Basic Camera

Stuck Filter Removal

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Filter rings are generally made from either aluminum or brass. Lens barrels, particularly the threads to which filters attach, are usually made from aluminum.

Filter rings, particularly aluminum ones, can sometimes "bind" to the aluminum lens threads and be difficult to remove.

Aluminum is a relatively soft metal; attempting to remove a stuck filter by squeezing with the hand generally puts a lot of inward pressure on just the two areas being gripped; this can bend and deform both the filter ring and the lens threads, permanently weakening or damaging both and making the filter even more difficult to remove.

Methods should be employed that apply pressure evenly around the filter ring.

Typically this is achieved either by use of a filter wrench or by cupping the filter ring and front of the lens with a piece of fabric to protect them and provide friction, then pressing the combination against a hard surface and twisting the lens barrel.

Other aids to stuck filter removal include using either a tightened rubber band or shoelace around the rim of the filter to improve grip.

Filter Sizes & Mountings - Bayonet Round Filters

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Certain manufacturers, most notably Rollei and Hasselblad, have created their own systems of bayonet mount for filters. Each design comes in several sizes, such as Bay I through Bay VIII for Rollei, and Bay 50 through Bay 104 for Hasselblad.

Filter Sizes & Mountings - Rectangular Filters

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Graduated filters of a given width (100 mm, 67 mm, 84 mm, etc.) are often made rectangular, rather than square, in order to allow the position of the gradation to be moved up or down in the picture. This allows, for example, the red part of a sunset filter to be placed at the horizon. These are used with the "system" holders described.

Filter Sizes & Mountings - Square Filters

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For square filters, 2" x 2", 3" x 3" and 4" x 4" were historically very common and are still made by some manufacturers. 100 mm x 100 mm is very close to 4"x4", allowing use of many of the same holders, and is one of the more popular sizes currently (2006) in use; it is virtually a standard in the motion picture industry. 75 mm x 75 mm is very close to 3" x 3" and while less common today, was much in vogue in the 1990s.

A French manufacturer called Cokin makes a wide range of filters and holders in three sizes which is collectively known as the Cokin System. "A" (amateur) size is 67 mm wide, "P" (professional) size is 84 mm wide, and "X Pro" is 130 mm wide. Many other manufacturers make filters to fit Cokin holders. Cokin also makes a filter holder for 100 mm filters, which they call the "Z" size. Most of Cokin's filters are made of optical resins such as CR-39. A few round filter elements may be attached to the square/rectangular filter holders, usually polarizers and gradient filters which both need to be rotated and are more expensive to manufacture.

Cokin formerly (1980s through mid-1990s) had competition from Hoya's Hoyarex system (75 mm x 75 mm filters mostly made from resin) and also a range made by Ambico, but both have withdrawn from the market. A small "system" range is still made (as of 2005) by Hitech. In general, square (and sometimes rectangular) filters from one system could be used in another system's holders if the size was correct, but each made a different system of filter holder which could not be used together. Lee, Tiffen and Singh Ray also make square / rectangular filters in the 100 x 100 and Cokin "P" sizes.

Gel filters are very common in square form, rarely being used in circular form. These are thin flexible sheets of plastic which must be held in rigid frames to prevent them from sagging. Gels are made not only for use as photo filters, but also in a wide range of colors for use in lighting applications, particularly for theatrical lighting. Gel holders are available from all of the square "system" makers, but are additionally provided by many camera manufacturers, by manufacturers of gel filters, and by makers of expensive professional camera accessories (particularly those manufacturers which target the movie and television camera markets.

Square filter systems often have lens shades available to attach to the filter holders.

Filter Sizes & Mountings - Threaded Round Filters

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The most common standard filter sizes for circular filters include 30.5 mm, 37 mm, 40.5 mm, 43 mm, 46 mm, 49 mm, 52 mm, 55 mm, 58 mm, 62 mm, 67 mm, 72 mm, 77 mm, 82 mm, 86 mm, 95 mm, 112 mm and 127 mm.

Other filter sizes within this range may be hard to find since the filter size may be non-standard or may be rarely used on camera lenses.

The specified diameter of the filter in millimeters indicates the diameter of the male threads on the filter housing. The thread pitch is 0.5 mm, 0.75 mm or 1.0 mm, depending on the ring size.

A few sizes (e.g. 30.5 mm) come in more than one pitch.

Filter diameter for a particular lens is commonly identified on the lens face by the ligature "ø". For example, a lens marking may indicate "ø 55mm."

Photo Filter - Materials & Construction

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Photo filters are commonly made from glass, resin plastics similar to those used for eyeglasses (such as CR39), polyester and polycarbonate; sometimes acetate is used. Historically, filters were often made from gelatin, and color gels, also called gelatin or simply gel filters are still used, but these are no longer actually made from gelatin, generally being instead made from one of the plastics mentioned above.

Sometimes a color is blended throughout the filter material, in other cases the filter is a sandwich composed of a thin sheet of material surrounded and supported by two pieces of clear glass or plastic.

Certain kinds of filters use other materials inside a glass sandwich; for example, polarizers often use various special films, netting filters have nylon netting, and so forth.

The rings on screw-on filters are most often made of aluminum, though in more expensive filters brass is used. Aluminum filter rings are much lighter in weight, but can "bind" to the aluminum lens threads they are screwed in to, requiring the use of a filter wrench to get the filter off of the lens. Aluminum also dents or deforms more easily. (See "Stuck filter removal" below.)

High quality filters have multiple layers of optical coating to reduce reflections and to allow more light to pass through the filter. Uncoated filters can block up to 9% of the light, while multi coated filters can allow for up to 99.7% of the light to pass through. Manufacturers brand their high-end multi coated filters with different labels, for example:

Hoya: HMC (Hoya Multi Coating)
B+W: MRC (Multi Resistant Coating)

Reflections can lead to flare and reduced contrast. Multi-layer coatings, which reduce this effect, are highly desirable in any filter. Exceptions to this rule are infrared and ultraviolet photography, where uncoated filters are usually used; multi-coated filters have a tendency to reflect more wavelengths outside the visible spectrum, making them unsuitable for such purposes.

Diopters & Split Diopters

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Some argument could be made as to whether these are technically filters at all, or actual accessory lenses, however they are sold by filter manufacturers as part of their product lines, using the same holders and attachment systems.

Diopters are simple single or two-element lenses used to assist in close-up and macro photography.

They provide some number of positive optical diopters, which magnify the subject and allow objects very close to the lens to be brought into focus.

They are sometimes sold singly, and sometimes sold in kits of +1, +2, and +4 diopters, which allows them to be combined to produce a range from +1 to +7.

A split diopter is a diopter in which only half of the camera's lens area is covered by the filter.

A round split diopter has a usual filter ring, but is filled with only a semicircle of glass (or plastic).

This allows the photographer to photograph an object which is very close against a background much further away, effectively extending depth of field.

Careful composition is required to make effective use of this device.

Grid or Netting

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Various widths, colors (often black or white), and grid shapes (typically diamonds or squares) and spacings of netting, usually made from nylon, are used to provide diffusion effects.

These are used both for the "dreamy" look and for contrast reduction.

The homebrew approach to this sort of effect is generally to stretch a piece of pantyhose material in front of the lens.

Transparent Diffusion

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Zeiss manufactures a widely noted Softar diffusion filter which is made of many tiny globs of acrylic deposited on one surface which act as microlenses to diffuse the light.

In some versions the globs are on the inside of the filter (facing the photographer) while on others they face outwards (towards the subject).

In various versions the globs vary in number and diameter, from approximately 97 to 150 globs each 1 mm to 3 mm wide.

Homebrew approaches to transparent diffusion filters are generally based on modifying a clear or UV filter by placing various materials on it; the most popular choices are petroleum jelly, optical cement, and nail polish.

Transparent filters are more commonly used for the "dreamy" or "misty" effect than for contrast reduction.

Diffusion

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A diffusion filter (also called a softening filter) softens subjects and generates a dreamy haze (see photon diffusion). This is most often used for portraits.

However, this also has the effect of reducing contrast, and the filters are designed, labeled, sold, and used for that purpose too.

There are many ways of accomplishing this effect, and thus filters from different manufacturers vary significantly.

The two primary approaches are to use some form of grid or netting in the filter, or to use something which is transparent but not optically sharp.

Both effects can be achieved in software, which can provide a very precise degree of control of the level of effect, however the "look" may be noticeably different.

Additionally, if there is too much contrast in a scene, the dynamic range of the digital image sensor or film may be exceeded, which post-processing cannot compensate for, so contrast reduction at the time of image capture may be called for.

Cross Screen

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A cross screen filter, also known as a star filter, creates a star pattern, in which lines radiate outward from bright objects.

The star pattern is generated by a very fine diffraction grating embedded in the filter, or sometimes by the use of prisms in the filter.

The number of stars varies by the construction of the filter, as does the number of points each star has.

Neutral Density

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A Neutral Density (ND) filter creates a reduction in light that is neutral and equal for the film or sensor area. This filter is often used to allow for longer exposure times whenever a longer exposure would normally create overexposure in the camera.

A Graduated Neutral Density (GND) filter is a neutral density filter that varies the effect with a gradient so it can be used to compress dynamic range across the entire scene. This can be beneficial when the difference between highlights and shadows of a scene are too great to allow for proper exposure for both.

Polarizer

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A polarizing filter, used both in color and black and white photography, can be used to darken overly light skies. Because the clouds are relatively unchanged, the contrast between the clouds and the sky is increased. Atmospheric haze and reflected sunlight are also reduced, and in color photographs overall color saturation is increased. Polarizers are often used to deal with situations involving reflections, such as those involving water or glass, including pictures taken through glass windows (this uses the phenomenon of Brewster's angle).

Polarizers are the type of filter whose use is least affected by digital photography; while effects that may visually resemble the results of a polarizing filter can be simulated with software post-processing, many of the optical properties of polarization control at the time of capture simply cannot be replicated, particularly those involving reflections.

The effects of a polarizer on the sky in a color photograph.
The picture on the right has the filter.

In the first picture, the polarizer is rotated to minimise the effect,
and in the second it is rotated 90° to maximize the effect
- almost all reflected sunlight is eliminated.

On the bottom you can see how the window is reflecting
the outside environment, while on the top the filter
is turned 90° making it possible to see
through the window.

There are two types of polarizing filters generally available, linear polarizers and circular polarizers (or CPL filters). With the exception of how they interact with some autofocus and metering mechanisms, they have exactly the same effect. Both transmit one of two states of linearly polarized light.

The difference is that a circular polarizer alters the light leaving the filter and entering the camera by using a quarter-wave plate to circularly polarize that light. This has the same effect photographically as a linear polarizer, reducing glare in the scene

The metering and auto-focus sensors in certain cameras, including virtually all auto-focus SLRs, will not work properly with linear polarizers because the beam-splitters used to split off the light for focusing and metering are polarization-dependent. Circular polarizers work with all types of cameras, because mirrors and beam-splitters reflect both circular polarizations equally.

Contrast Enhancement

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Filters are commonly used in black and white photography to manipulate contrast. For example a yellow filter will enhance the contrast between clouds and sky by darkening the latter. Orange and red filters will have a stronger effect. A deep green filter will darken the sky too but will lighten green foliage and will make it stand out against the sky. Also see diffusion filters, which are used to reduce contrast.

Effects of using a polarizer and a red filter in
black-and-white photography.

Color Subtraction

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Color subtraction filters work by absorbing certain colors of light, letting the remaining colors through.

They can be used to demonstrate the primary colors that make up an image.

They are perhaps most frequently used in the printing industry for color separations, and again, use has diminished as digital solutions have proliferated.

Color Correction

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A major use is to compensate the effects of lighting not balanced for the film stock's rated color temperature (usually 3200 K for professional tungsten lights and 5500 K for daylight): e.g., the 80A blue filter used with daylight film corrects the orange/reddish cast of household tungsten lighting, while the 85B used with tungsten film will correct the bluish cast of daylight. Color correction filters are identified by numbers which sometimes vary from manufacturer to manufacturer. The use of these filters has been greatly reduced by the widespread adoption of digital photography, since color balance problems are now often addressed with software after the image is captured.

Although the 80A filter is mainly used to correct for the excessive
redness of tungsten lighting, it can also be used to
oversaturate scenes that already have blue.
The photo on the left was shot with
a polarizer, while the
one on the right was
shot with a polarizer
and an 80A filter.

Clear & Ultraviolet

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Clear filters, also known as window glass filters or optical flats, are completely transparent, and (ideally) perform no filtering of incoming light at all. The only use of a clear filter is to protect the front of a lens.

UV filters are used to reduce haziness created by ultraviolet light. A UV filter is mostly transparent to visible light, and can be left on the lens for nearly all shots. UV filters are often used for lens protection, much like clear filters. A strong UV filter, such as a Haze-2A or UV17, cuts off some visible light in the violet part of the spectrum, and so has a pale yellow color; these strong filters are more effective at cutting haze, and can reduce purple fringing in digital cameras. Strong UV filters are also sometimes used for warming color photos taken in shade with daylight-type film.

While in certain cases (such as harsh environments) a protection filter may be necessary, there are also downsides to this practice. Arguments for and against use of protection filters incude:

For:

If the lens is dropped, the filter may well suffer scratches or breakage instead of the front lens element.
One can clean the filter frequently without having to worry about damaging the lens coatings; a filter scratched by cleaning is much less expensive to replace than a lens.

Against:

Adding another element degrades image quality due to aberration and flare.
It may reduce the use of lens hoods, since threading a lens hood on top of the clear filter might cause vignetting on some lenses, and since not all clear filters would even have threads allowing a hood to be attached.

Additionally, users of UV filters must be careful about the quality of such filters. There is a wide variance in the performance of these filters with respect to their ability to block UV light. Also in lower quality filters, problems with autofocus and image degradation have been noted.

Uses of filters in photography

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Filters in photography can be classified according to their use:

Clear and ultraviolet
Color correction, also called "color conversion" or "white balance correction"
Color separation, also called Color Subtraction
Contrast enhancement
Infrared
Neutral Density, including the Graduated ND filter and Solar filter
Polarizing
Special Effects of various kinds, including
- Graduated color, called color grads
- Cross screen and Star diffractors
- Diffusion and contrast reduction
- Sepia tone
- Spot
- Close-up or macro diopters, and split diopters or split focus

Photographic Filter

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In photography, a filter is a camera accessory consisting of an optical filter that can be inserted in the optical path. The filter can be a square or rectangle shape mounted in a holder accessory, or, more commonly, a glass or plastic disk with a metal or plastic ring frame, which can be screwed in front of the lens.

Filters allow added control for the photographer of the images being produced. Sometimes they are used to make only subtle changes to images; other times the image would simply not be possible without them.

The negative aspects of using filters, though often negligible, include the possibility of loss of image definition if using dirty or scratched filters, and increased exposure required by the reduction in light transmitted. The former is best avoided by careful use and maintenance of filters, while the latter is a matter of technique; it usually will not be a problem if planned out properly, but in some situations does make filter use impractical.

Many filters are identified by their Wratten number (labeling system for optical filters).

62 mm ultraviolet, polarizing,
and fluorescent lens filters.

Environmental & Safety Issues

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Many photographic solutions have high chemical and biological oxygen demand (COD and BOD). These chemical wastes are often treated with ozone, peroxide or aeration to reduce the COD.

Exhausted fixer and to some extent rinse water contain silver-thiosulfate complex ions. They are far less toxic than free silver ion, and they become silver sulfide sludge in the sewer pipes or treatment plant. However, the maximum silver concentration in discharge is very often tightly regulated. Silver is also a somewhat precious resource. Therefore, in most large scale processing establishments, exhausted fixer is collected for silver recovery and disposal.

Many photographic chemicals use non-biodegradable compounds, such as EDTA, DTPA, NTA and borate. EDTA, DTPA, and NTA are very often used as chelating agents in all processing solutions, particularly in developers and washing aid solutions. EDTA and other polyamine polycarboxylic acids are used as iron ligands in color bleach solutions. These are relatively nontoxic, and in particular EDTA is approved as a food additive. However, due to poor biodegradability, these chelating agents are found in alarmingly high concentrations in some water sources from which municipal tap water is taken. Water containing these chelating agents can leach metal from water treatment equipment as well as pipes. This is becoming an issue in Europe and some parts of the world.

Another non-biodegradable compound in common use is surfactant. A common wetting agent for even drying of processed film uses Union Carbide/Dow Triton X-100 or octylphenol ethoxylate. This surfactant is also found to have estrogenic effect and possibly other harms to organisms including mammals.

Development of more biodegradable alternatives to the EDTA and other bleaching agent constituents were sought by major manufacturers, until the industry became less profitable when the digital era began.

In most amateur darkrooms, a popular bleach is potassium hexacyanoferrate (III) (common name potassium ferricyanide). This compound decomposes in the waste water stream to liberate cyanide gas. Other popular bleach solutions use potassium dichromate (a hexavalent chromium) or permanganate. Both ferricyanide and dichromate are tightly regulated for sewer disposal. In order to meet the regulation, the solution must be diluted 20 000 (twenty thousand) times or more. All of these popular black and white bleaches are damaging to the environment, and should be replaced with any of the existing eco-friendly alternatives.

Borates, such as borax (sodium tetraborate), boric acid and sodium metaborate, are toxic to plants, even at a concentration of 100 ppm. Many film developers and fixers contain 1 to 20 g/L of these compounds at working strength. Most non-hardening fixers from major manufacturers are now borate-free, but many film developers still use borate as the buffering agent. Also, some, but not all, alkaline fixer formulas and products contain a large amount of borate. New products should phase out borates, because for most photographic purposes, except in acid hardening fixers, borates can be substituted with a suitable biodegradable compound.

Developing agents are commonly hydroxylated benzene compounds or aminated benzene compounds, and they are harmful to humans and experimental animals. Some are mutagens. They also have a large chemical oxygen demand (COD). Ascorbic acid and its isomers, and other similar sugar derived reductone reducing agents are a viable substitute for many developing agents. Developers using these compounds were actively patented in the US, Europe and Japan, until 1990s but the number of such patents is very low since late-1990s, when the digital era began.

Further Processing

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For black and white emulsions both negative and positive, further processing may react the silver with other elements such as selenium or sulfur to produce a more permanent image.

In these cases the silver in the image is changed to silver selenide or silver sulfide, which are more resistant to oxidising agents (pollutants) in the atmosphere.

Successful selenium or sulfide toning requires complete fixation. In other cases, the silver may be chemically bleached using a potassium hexacyanoferrate (III) solution and then re-developed in a range of toning formulations.

This two-step technique is collectively called "indirect toning." Depending on the toning solution, sepia, red and blue colors may be obtained.

If color negative film is processed in conventional black and white developer, and fixed and then bleached with a bath containing hydrochloric acid and potassium dichromate solution, the resultant film, once exposed to light, can be redeveloped in color developer to produce an unusual pastel color effect.

Color Processing

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Color negative developing (C-41 process) and color print developing (process RA-4) are very similar. The principal difference is in the color developer formula in the first step; and the combining of the bleach and fixer steps in process RA-4 with a bleach-fix mixture (blix), which dissolves both the silver halides and the elemental silver leaving only a dye image.

Mixing the bleach and fixing agent in process RA-4 is optional; they are often mixed to minimize the number of processing steps.

In color reversal processing the film is developed in an MQ (metol/hydroquinone) developer similar to a black-and-white developer, followed by a rinse or a stop bath. The film is fogged in the reversal step, and is then developed in a color developer.

Next, the film is then bleached to remove the black-and-white negative image (metallic silver is removed by the bleach), while keeping the color positive image (dye is not affected by the bleach).

The film is optionally washed to minimize the carry-over of the bleach to the next bath, fixer. Film is then fixed, washed and dried conventionally. In some old processes, the film emulsion was hardened during the process, typically before the bleach. Such a hardening bath often used aldehydes, such as formaldehyde and glutaraldehyde.

In modern processing, these hardening steps are unnecessary because the film emulsion is sufficiently hardened to withstand the processing chemicals.

Monochrome (Black & White) Reversal Processing

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Black and white reversal processing (to make black and white positives) has two additional stages. Following the first developer, the developing action is stopped by an ordinary stop bath, and the film is then bleached to remove the negative image produced by development.

The film then contains a latent positive image in the form of unexposed, undeveloped silver halide. The subsequent steps therefore fog the film, either chemically, or by exposure to light, after which all the remaining silver halide is developed in the second developer.

This allows the silver halide crystals that were not originally exposed in the camera to be developed into a positive image. Finally, the film is fixed, washed and dried conventionally.

The Process

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Whether processing at an amateur or commercial level, the film is treated in chemical baths. Each of these baths is closely monitored and maintained at a specific temperature and treatment time. Developer baths are most sensitive to deviations from the standard processing conditions (e.g., time and temperature); other baths are less sensitive.

The developer, which turns the latent image to metallic silver.
A stop bath, which stops the action of developer, typically a dilute solution of acetic acid. In modern automatic processing machines, this step is replaced by mechanical squeegee or pinching rollers. In small scale darkrooms, stop bath may use citric acid or other organic acids, or simply plain water. Any of these treatments removes the bulk of the carried-over alkaline developer, and the acid, when used, neutralizes the alkalinity to reduce the contamination of the fixing bath with the developer.
The fixer makes the image permanent and light-resistant by dissolving any remaining silver halides. The fixer is sometimes referred to as "hypo," a misnomer originating from casually shortened form of the alchemist's name hyposulphite. None of "hyposulphite," "hyposulfite" and "hypo" is used to mean thiosulfate in modern chemistry.
Clean water wash to remove any fixer, as residual fixer can deteriorate the silver image, leading to discoloration, staining and fading. The washing time can be reduced and the fixer more completely removed if a washing aid (also known as hypo clearing agent, or HCA) is used after the fixer.
An optional final immersion in a film conditioner. This is a solution of a non-ionic wetting agent in water which helps uniform drying and eliminates drying marks from hard water.
Film is then dried in a dust-free environment, and finally cut (if roll film) and put into protective sleeves.

Processing Apparatuses

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Before processing, the film must be removed from the camera and from its cassette, spool or holder in a light-proof room or container.

Small scale processing

In amateur processing, the film is removed from the camera and wound onto a reel in complete darkness (usually inside a darkroom (with the safelight turned off) or a lightproof bag with arm holes). The reel holds the film in a spiral shape, with space between each successive loop so the chemicals may flow freely across the film's surfaces. The reel is placed in a specially designed light-proof tank (called daylight processing tank) where it is retained until final washing is complete.

In case of sheet film, they can be processed in trays, in hangers (which are used in deep tanks), or rotary processing drums.

Commercial processing

In commercial processing, the film is removed automatically or by an operator handling the film in a light proof bag from which it is fed into the processing machine. The processing machinery is generally run on a continuous basis with films spliced together in a continuous line. All the processing steps are carried out within a single processing machine with automatically controlled time, temperature and solution replenishment rate. The film or prints emerge washed and dry and ready to be cut by hand. Some modern machines also cut films and prints automatically, sometimes resulting in negatives cut across the middle of the frame where the space between frames is very thin or the frame edge is indistinct, as in an image taken in low light.

Photographic Processing

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Photographic processing is the industrial process by which conventional photographic film is treated after photographic exposure in order to produce the desired negative or positive image.

Photographic processing does three things: it transforms the latent image into a visible image that can be seen, it makes the visible image permanent, and it renders the film insensitive to light.

The general process is similar whatever the make of film or paper. Although generally not considered "conventional," exceptions include instant films such as Polaroid and thermally developed films.

Kodachrome cannot be processed except in Kodak laboratories. There are also a small number of relatively uncommon processes using dye decomposition technologies such as Cibachrome.

Manufacturer of Photographic Film

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*: discontinued, **: bankruptcy

AgfaPhoto**
Agfa-Gevaert (Materials division)
Bergger (European company composed of former Guilleminot employees.)
Dai Nippon Printing
Efke
Foma
Forte
Ferrania

Fujifilm
Ilford
Imation (Spin-off company of 3M has since sold the film business to Ferrania)
Kodak
Konica Minolta*
Lucky
Maco
Mitsubishi

ORWO
Perutz
Polaroid*
ProClick
Solaris (Ferrania)
Svema
Tasma
Tura

Film manufacturers commonly make film that is branded by other companies. Modern films have bar codes on the edge of the film which can be read by a bar code reader. This is because film is sometimes processed differently according to specifications of the film, determined by its manufacturer; the bar code is entered into the computer printer before the film is printed.

To establish the OEM, read the bar code printed on the cassette. Divide the long number by 16 and record the number before the decimal, then multiply the number after the decimal by 16, this could give you a result such as 18 and 2.

The first number is known as the PRODUCT (film manufacturer) and the second number as the MULTIPLIER (speed of the film ISO). In the previous example, 18 identifies 3M as the manufacturer and 2 means it is 200 ISO:

3M = 18
Agfa = 17 or 49
Kodak = 80, 81, 82 or 88

Common Sizes of Film

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135 (popularly known as "35 mm")
APS (Advanced Photo System)
110
126
127
120/220 (for use in medium format photography)
Sheet film (for use in large format photography)
Disc film Obsolete format used in disc system cameras
Motion picture films: 8 mm, 16 mm, 35 mm and 70 mm

Special Films

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Instant photography, as popularised by Polaroid, uses a special type of camera and film that automates and integrates development, without the need of further equipment or chemicals. This process is carried out immediately after exposure, as opposed to regular film, which is developed afterwards and requires additional chemicals. See instant film.

Films can be made to record non-visible ultraviolet (UV) and infrared (IR) radiation. These films generally require special equipment; for example, most photographic lenses are made of glass and will therefore filter out most ultraviolet light. Instead, expensive lenses made of quartz must be used. Infrared films may be shot in standard cameras using an infrared band- or long-pass filter, although the infrared focal point must be compensated for.

Exposure and focusing are difficult when using UV or IR film with a camera and lens designed for visible light. The ISO standard for film speed only applies to visible light, so visual-spectrum light meters are nearly useless. Film manufacturers can supply suggested equivalent film speeds under different conditions, and recommend heavy bracketing. e.g with a certain filter, assume ISO 25 under daylight and ISO 64 under tungsten lighting.

This allows a light meter to be used to estimate an exposure. The focal point for IR is slightly father away from the camera than visible light, and UV slightly closer; this must be compensated for when focussing. Apochromatic lenses are sometimes recommended due to their improved focusing across the spectrum.

Film optimized for sensing X-ray radiation is commonly used for medical imaging by placing the subject between the film and a source of X-rays, without a lens, as if a translucent object were imaged by being placed between a light source and standard film.

Film optimized for sensing X-rays and for gamma rays is sometimes used for radiation dosimetry and personal monitoring.

Film has a number of disadvantages as a scientific detector: it is difficult to calibrate for photometry, it is not re-usable, it requires careful handling (including temperature and humidity control) for best calibration, and the film must physically be returned to the laboratory and processed.

Against this, photographic film can be made with a higher spatial resolution than any other type of imaging detector, and, because of its logarithmic response to light, has a wider dynamic range than most digital detectors. For example, Agfa 10E56 holographic film has a resolution of over 4,000 lines/mm—equivalent to a pixel size of 0.125 micrometres—and an active dynamic range of over five orders of magnitude in brightness, compared to typical scientific CCDs that might have pixels of about 10 micrometres and a dynamic range of 3-4 orders of magnitude.

Special films are used for the long exposures required by astrophotography.

History of Film

2008-06-27T01:42:00.001+08:00

Hurter & Driffield began pioneering work on the light sensitivity of film in 1876 onwards. Their work enabled the first quantitative measure of film speed to be devised.

Early photography in the form of daguerreotypes did not use film at all. Eastman Kodak developed the first flexible photographic film in 1885. This original "film" was coated on paper. The first transparent plastic film was produced in 1889. Before this, glass photographic plates were used, which were far more expensive and cumbersome, albeit also of better quality.

The first photographic film was made from highly flammable nitrocellulose with camphor as a plasticizer (celluloid). Beginning in the 1920s, nitrate film was replaced with cellulose acetate or "safety film". This changeover was not completed until 1933 for X-ray films (where its flammability hazard was most acute) and for motion picture film until 1951.

Spectral sensitivity

The first films were sensitive to blue light only. Orthochromatic film sensitive to the spectral range from green to blue was introduced in 1879 and was dominant until the mid-1920s, when panchromatic film sensitive to the entire visual spectrum, became standard. All of these films were used to produce black and white images, regardless of spectral sensitivity.

Experiments with color photography were first made in 1861, but generally usable emulsions only became available in the 1930s. After World War II much progress was made, and color became used for the overwhelming majority of photographs.

Effect on lens and equipment design

Photographic lenses and equipment are designed around the film to be used. The earliest lenses needed to focus blue light only. The introduction of orthochromatic film required the spectrum from green to blue to be brought to the same focus. A red window could be used to view frame numbers of rollfilm; any red light which leaked beyond the film backing would not fog the film; and red lighting could be used in darkrooms.

With the introduction of panchromatic film the whole visual spectrum needed to be brought to the same focus. In all cases a color cast in the lens glass or faint colored reflections in the image were of no consequence as they would merely change the contrast a little. This was no longer acceptable with the introduction of color film. More highly corrected lenses for newer emulsions could be used with older emulsion types, but the converse was not true.

The filters used were different for the different film types.

The progression of lens design for later emulsions is of practical importance when considering the use of old lenses, still often used on large-format equipment; a lens designed for orthochromatic film may have visible defects with a color emulsion; a lens for panchromatic film will be better but not as good as later designs.

While color processing is more complex and temperature-sensitive than for monochromatic film, the great popularity of color and almost disappearance of monochrome prompted the design of monochromatic film which is processed in exactly the same way as a standard color film.

Film Speed

2008-06-27T01:36:00.002+08:00

Film speed describes a film's threshold sensitivity to light. The international standard for rating film speed is the ISO scale which combines both the ASA speed and the DIN speed in the format ASA/DIN. Using ISO convention film with an ASA speed of 400 would be labeled 400/27°. A fourth naming standard is GOST, developed by the Russian standards authority.

Common film speeds include ISO 25, 50, 64, 100, 160, 200, 400, 800, 1600, and 3200. Consumer print films are usually in the ISO 100 to ISO 800 range.

Some films, like Kodak's Technical Pan, are not ISO rated and therefore careful examination of the film's properties must be made by the photographer before exposure and development.

ISO 25 film is very "slow", as it requires much more exposure to produce a usable image than "fast" ISO 800 film. Films of ISO 800 and greater are thus better suited to low-light situations and action shots (where the short exposure time limits the total light received).

The benefit of slower films is that it usually has finer grain and better color rendition than fast film. Professional photography with static subjects such as portraits or landscapes usually seek these qualities, and therefore require a tripod to stabilize the camera for a longer exposure.

Photographing subjects such as rapidly moving sports or in low-light conditions, a professional will choose a faster film. Grain size refers to the size of the silver crystals in the emulsion. The smaller the crystals, the finer the detail in the photo and the slower the film.

A film with a particular ISO rating can be pushed to behave like a film with a higher ISO. In order to do this, the film must be developed for a longer amount of time or at a higher temperature than usual.

This procedure is usually only performed by photographers who do their own development or professional-level photofinishers. More rarely, a film can be pulled to behave like a "slower" film.

Film Basics

2008-06-27T01:32:00.001+08:00

There are two primary types of photographic film:

Print film, when developed, turns into a negative with the colors (or black and white values, in black and white film) inverted. This type of film must be "printed" — either projected through a lens or placed in contact — to photographic paper in order to be viewed as intended. Print films are available in both black-and-white and color.

Color reversal film after development is called a transparency and can be viewed directly using a loupe or projector. Reversal film mounted with plastic or cardboard for projection is often called a slide. It is also often marketed as "slide" film. This type of film is often used to produce digital scans or color separations for mass-market printing. Photographic prints can be produced from reversal film, but the process is expensive and not as simple as that for print film. Black and white reversal film exists, but is uncommon—one of the reasons reversal films are popular among professional photographers is the fact that they are generally superior to print films with regards to color reproduction. (Conventional black and white negative stock can be reversal-processed, to give "black & white slides", and kits are available to enable this to be done by home-processors. As indicated by Grant Haist's published book Modern Photographic Processing, B&W transparencies can be produced from most all B&W films.)

In order to produce a usable image, the film needs to be exposed properly. The amount of exposure variation that a given film can tolerate while still producing an acceptable level of quality is called its exposure latitude. Color print film generally has greater exposure latitude than other types of film. Additionally, because print film must be printed to be viewed, after-the-fact corrections for imperfect exposure are possible during the printing process.

The concentration of dyes or silver salts remaining on the film after development is referred to as optical density, or simply density; the optical density is proportional to the logarithm of the optical transmission coefficient of the developed film. A dark image on the negative is of higher density than a more transparent image.

Most films are affected by the physics of silver grain activation (which sets a minimum amount of light required to expose a single grain) and by the statistics of random grain activation by photons. The film requires a minimum amount of light before it begins to expose, and then responds by progressive darkening over a wide dynamic range of exposure until all of the grains are exposed and the film achieves (after development) its maximum optical density.

Over the active dynamic range of most films, the density of the developed film is proportional to the logarithm of the total amount of light to which the film was exposed, so the transmission coefficient of the developed film is proportional to a power of the reciprocal of the brightness of the original exposure. This is due to the statistics of grain activation: as the film becomes progressively more exposed, each incident photon is less likely to impact a still-unexposed grain, yielding the logarithmic behavior. A simple, idealized statistical model yields the equation density = 1 - ( 1 - k ) ^ light, where "light" is proportional to the number of photons hitting a unit area of film, "k" is the probability of a single photon striking a grain (based on the size of the grains and how closely spaced they are), and density is the proportion of grains that where hit by at least one photon.

If parts of the image are exposed heavily enough to approach the maximum density possible for a print film, then they will begin losing the ability to show tonal variations in the final print. Usually those areas will be deemed to be overexposed and will appear as featureless white on the print. Some subject matter is tolerant of very heavy exposure; brilliant light sources like a bright lightbulb, or the sun, included in the image generally appear best as a featureless white on the print.

Likewise, if part of an image receives less than the beginning threshold level of exposure, which depends upon the film's sensitivity to light—or speed—the film there will have no appreciable image density, and will appear on the print as a featureless black. Some photographers use their knowledge of these limits to determine the optimum exposure for a photograph; for one example, see the Zone system. Most automatic cameras instead try to achieve a particular average density.

Photographic Film

2008-06-27T01:22:00.003+08:00

Photographic film is a sheet of plastic (polyester, nitrocellulose or cellulose acetate) coated with an emulsion containing light-sensitive silver halide salts (bonded by gelatin) with variable crystal sizes that determine the sensitivity, contrast and resolution of the film. When the emulsion is sufficiently exposed to light (or other forms of electromagnetic radiation such as X-rays), it forms a latent (invisible) image. Chemical processes can then be applied to the film to create a visible image, in a process called film developing.

In black-and-white photographic film there is usually one layer of silver salts. When the exposed grains are developed, the silver salts are converted to metallic silver, which block light and appear as the black part of the film negative.

Color film uses at least three layers. Dyes, which adsorb to the surface of the silver salts, make the crystals sensitive to different colors. Typically the blue-sensitive layer is on top, followed by the green and red layers. During development, the exposed silver salts are converted to metallic silver, just as with black and white film.

But in a color film, the by-products of the development reaction simultaneously combine with chemicals known as color couplers that are included either in the film itself or in the developer solution to form colored dyes. Because the by-products are created in direct proportion to the amount of exposure and development, the dye clouds formed are also in proportion to the exposure and development.

Following development, the silver is converted back to silver salts in the bleach step. It is removed from the film in the fix step. This leaves behind only the formed color dyes, which combine to make up the colored visible image.

Newer color films, like Kodacolor II, have as many as 12 emulsion layers, with upwards of 20 different chemicals in each layer.

Because photographic film is widespread in the production of motion pictures, or movies, these are also known as films.

Camera & Mount For AF Lens

2008-06-24T13:45:00.004+08:00

Camera: Minolta
Mount: Minolta Mount

Camera: Nikon
Mount: F Mount

Camera: Pentax
Mount: K Mount

Camera: Pentax AF
Mount: KAF Mount

Camera: Ricoh
Mount: KAF Mount

(Chonon & Topcon also can use Pentax's Mount)

Camera: Yashica
Mount: Yashica / Contax Mount

Camera & Lens For Mechanical SLR

2008-06-24T13:40:00.002+08:00

Camera: Nikon
Lens Name: Nikkor

Camera: Minolta
Lens Name: MD Rokkor

Camera: Canon
Lens Name: Canon

Camera: Olympus
Lens Name: Zaiko

Camera: Pentax
Lens Name: Takumar

Camera: Mamiya
Lens Name: Sekor

Camera: Bronica
Lens Name: Zenzinon

Camera: Ricoh
Lens Name: Rikenon

Camera: Topkon
Lens Name: Topco R

List of Lens Mount Types

2008-06-24T13:37:00.000+08:00

Mount Name: D mount
Camera Type: 8 mm movie and CCTV
Mount Type: Screw (0.625 inch × 32 TPI)
Flange Focal Distance: 12.29 mm

Mount Name: CS mount
Camera Type: 8 mm movie and CCTV
Mount Type: Screw (1 inch × 32 TPI)
Flange Focal Distance: 12.52 mm

Mount Name: C mount
Camera Type: 8 mm movie and CCTV
Mount Type: Screw (1 inch × 32 TPI)
Flange Focal Distance: 17.526 mm (0.69 inches)

Mount Name: Canon EX
Camera Type: Camcorder
Mount Type: Bayonet
Flange Focal Distance: 20 mm

Mount Name: Bole
Camera Type: 16 mm movie
Mount Type: Breech lock
Flange Focal Distance: 23.22 mm

Mount Name: Leica M bayonet
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 27.8 mm

Mount Name: Canon screw mount
Camera Type: 35 mm still
Mount Type: Screw (M39 × 1 mm)
Flange Focal Distance: -

Mount Name: Leica M39 screw mount
Camera Type: 35 mm still, enlargers
Mount Type: Screw (M39 × 26 TPI)
Flange Focal Distance: 28.8 mm

Mount Name: Narcissus
Camera Type: 35 mm still
Mount Type: Screw (M24 × 1 mm)
Flange Focal Distance: 28.8 mm

Mount Name: Olympus Pen F
Camera Type: 35 mm half-frame still
Mount Type: Bayonet
Flange Focal Distance: 28.95 mm

Mount Name: Hasselblad Xpan
Camera Type: 35 mm panoramic still
Mount Type: Bayonet
Flange Focal Distance: 34.27 mm

Mount Name: Four Thirds
Camera Type: digital still
Mount Type: Bayonet
Flange Focal Distance: 38.67 mm

Mount Name: Aaton universal
Camera Type: 16 mm movie
Mount Type: Breech lock
Flange Focal Distance: 40 mm

Mount Name: Canon R
Camera Type: 35 mm still
Mount Type: Breech lock
Flange Focal Distance: 42 mm

Mount Name: Canon FL
Camera Type: 35 mm still
Mount Type: Breech lock
Flange Focal Distance: 42 mm

Mount Name: Canon FD
Camera Type: 35 mm still
Mount Type: Breech lock
Flange Focal Distance: 42 mm

Mount Name: Fujica-X
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 43.5 mm

Mount Name: Minolta MC/MD
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 43.5 mm

Mount Name: Petriflex
Camera Type: 35 mm still
Mount Type: Breech lock
Flange Focal Distance: 43.5 mm

Mount Name: Canon EF
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 44 mm

Mount Name: Canon EF-S
Camera Type: APS-C digital still
Mount Type: Bayonet
Flange Focal Distance: 44 mm

Mount Name: Sigma SA
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 44 mm

Mount Name: Paxette
Camera Type: 35 mm still
Mount Type: Screw (M39×1 mm)
Flange Focal Distance: 44 mm

Mount Name: Praktica B
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 44.4 mm

Mount Name: Minolta AF
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 44.5 mm

Mount Name: Rolleiflex SL35
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 44.6 mm

Mount Name: Exakta, Topcon
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 44.7 mm

Mount Name: Pentax K
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 45.5 mm

Mount Name: M42 lens mount or Praktica
Camera Type: 35 mm still
Mount Type: Screw (42 mm×1mm)
Flange Focal Distance: 45.5 mm

Mount Name: Olympus OM
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 46 mm

Mount Name: Yashica / Contax
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 45.5 mm

Mount Name: Nikon F-mount
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 46.5 mm

Mount Name: Leica R
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 47 mm

Mount Name: Contax-N
Camera Type: 35 mm still
Mount Type: Bayonet
Flange Focal Distance: 48 mm

Mount Name: Praktina
Camera Type: 35 mm still
Mount Type: Breech lock
Flange Focal Distance: 50 mm

Mount Name: Arri standard
Camera Type: 35 mm and 16 mm movie
Mount Type: Tab lock
Flange Focal Distance: 52 mm

Mount Name: Arri bayonet
Camera Type: 35 mm and 16 mm movie
Mount Type: Bayonet
Flange Focal Distance: 52 mm

Mount Name: Arri PL
Camera Type: 35 mm and 16 mm movie
Mount Type: Breech lock
Flange Focal Distance: 52 mm

Mount Name: T-mount or T-thread
Camera Type: 35 mm still
Mount Type: Screw (M42×0.75)
Flange Focal Distance: 55 mm

Mount Name: PV mount
Camera Type: 35 mm movie
Mount Type: Breech lock
Flange Focal Distance: 57.15 mm

Mount Name: BNCR mount
Camera Type: 35 mm movie
Mount Type: Breech lock
Flange Focal Distance: 61.468 mm

Mount Name: Mamiya 645
Camera Type: Medium format
Mount Type: Bayonet
Flange Focal Distance: 63.3 mm

Mount Name: Leitz Visoflex II / III
Camera Type: 35 mm still
Mount Type: Bayonet (Leica M)
Flange Focal Distance: 68.8 mm

Mount Name: Pentax 645
Camera Type: Medium format still
Mount Type: Bayonet
Flange Focal Distance: 70.87 mm

Mount Name: Pentacon Six
Camera Type: Medium format still
Mount Type: Breech lock
Flange Focal Distance: 74.1 mm

Mount Name: Pentax 6x7
Camera Type: Medium format still
Mount Type: Bayonet
Flange Focal Distance: 84.95 mm

Mount Name: Leitz Visoflex I
Camera Type: 35 mm still
Mount Type: Screw (M39×26 TPI)
Flange Focal Distance: 91.3 mm

Mount Name: Mamiya RZ
Camera Type: Medium format still
Mount Type: Bayonet
Flange Focal Distance: 105 mm

Mount Name: Mamiya RB
Camera Type: Medium format still
Mount Type: Bayonet
Flange Focal Distance: 112 mm

Mount Name: Mamiya 7/7II
Camera Type: Medium format still
Mount Type: Bayonet
Flange Focal Distance: ~60 mm ???

Mount Name: Kowa Six/Super 66
Camera Type: Medium format still
Mount Type: Breech lock
Flange Focal Distance: 79 mm

Mount Name: Hasselblad
Camera Type: Medium format still
Mount Type: Bayonet
Flange Focal Distance: 74.9 mm

Mount Name: Bronica S2A
Camera Type: Medium format still
Mount Type: Bayonet & 57×1 thread
Flange Focal Distance: 101.7 mm

Mount Name: Rolleiflex SL66
Camera Type: Medium format still
Mount Type: Bayonet
Flange Focal Distance: 102.8 mm

Secondary Lens Mount

2008-06-20T09:43:00.005+08:00

Secondary lens refers to a multi-element lens mounted either in front of a camera's primary lens, or in between the camera body and the primary lens.

(D)SLR camera & interchangeable-lens manufacturers offer lens accessories like extension tubes and secondary lenses like teleconverters; which mount in between the camera body and the primary lens, both using and providing a primary lens mount.

Canon PowerShot A and Canon PowerShot G cameras have a built-in or non-interchangeable primary (zoom) lens; and Canon has "conversion tube" accessories available for some Canon PowerShot camera models which provides either a 52mm or 58mm "accessory/filter" screw thread.

Canon's close-up, wide - (WC-DC), and tele-conversion (TC-DC) lenses have 2, 3, and 4-element lenses respectively, so they are multi-element lenses and not diopter "filters".

List of Lens Mount Types

2008-06-20T01:06:00.004+08:00

Stills

Canon EF
Canon EF-S
Canon FD
Canon FL
Contax N
Contax/Yashica bayonet
Four Thirds System
Fujica X bayonet
Konica original bayonet
Konica AR 47mm bayonet
Leica M mount
Leica R bayonet
M42
Mamiya bayonet
Minolta AF
Minolta MD
Miranda bayonet (all Miranda cameras had a dual bayonet/M42 screw mount)
Nikon F
Olympus OM
Pentax K
Sigma SA
T-mount (T-thread)
Yashica AF

Cine

Aaton universal
Arri bayonet
Arri PL
Arri standard
BNCR
C mount
CA-1
PV (Panavision)

Industrial

C mount
CS mount
Front-plate mount

Lens Mount

2008-06-20T00:49:00.005+08:00

A lens mount is an interface — mechanical and often also electrical — between a photographic camera body and a lens. It is confined to cameras where the body allows interchangeable lenses, most usually the single lens reflex type or any movie camera of 16 mm or higher gauge. Lens mounts are also used to connect optical components in instrumentation that may not involve a camera, such as the modular components used in optical laboratory prototyping which join via C-mount or T-mount elements.

A Leica R series teleconverter, with
the female side of the Leica R
bayonet mount. This side
is also used on the
camera body.

A lens mount may be a screw-threaded type, a bayonet-type, or a friction lock type. Modern still camera lens mounts are of the bayonet type, because the bayonet mechanism precisely aligns mechanical and electrical features between lens and body. Screw-threaded mounts are fragile and do not align the lens in a reliable rotational position, yet types such as the C-mount interface are still widely in use for other applications like video cameras and optical instrumentation.

…and the male side.

Bayonet mounts generally have a number of tabs (often three) around the base of the lens, which fit into appropriately sized recesses in the lens mounting plate on the front of the camera. The tabs are often "keyed" in some way to ensure that the lens is only inserted in one orientation, often by making one tab a different size. Once inserted the lens is fastened by turning it a small amount. It is then locked in place by a spring-loaded pin, which can be operated to remove the lens.

Lens mounts of competing manufacturers (Nikon, Canon, Contax/Yashika, Pentax, etc.) are almost always incompatible. Many allege that this is due to the desire of manufacturers to "lock in" consumers to their brand. However, since there are other differences between manufacturers — specifically the flange focal distance from the lens mount to the film or sensor — one would not want to mount a lens which wasn't specifically designed for their type of camera, at least not without an adapter to correct the spacing.

In movie cameras, the two most popular mounts in current usage on professional 35 mm cameras are Arri's PL mount and Panavision's PV mount. The Panavision mounts are exclusively used with Panavision lenses, and thus are only available on Panaflex cameras or third-party cameras "Panavised" by a Panavision rental house, whereas the PL mount style is favored with most other cameras and cine lens manufacturers. Both of these mounts are held in place with locating pins and friction locking rings. Other mounts which are now largely historical or a minority in relation to current practices are listed below.

Lens Cover

2008-06-20T00:28:00.001+08:00

Lens cover or lens cap provides protection from scratches and minor collisions for camera and camcorder lenses. Lens covers come standard with most cameras and lenses. Some mobile camera phones include lens covers, such as the Sony Ericsson W800 and the Sony Ericsson K750.

Canon lens cover

Lens Hood

2008-06-19T23:50:00.004+08:00

Lens hood or lens shade is a device used on the end of a lens to block the sun or other light source in order to prevent glare and lens flare.

The geometry of the lens hood can vary from a plain conical section (much like a lamp shade) to a more complex cut sometimes called a flower, petal or tulip hood (as shown in the picture), which prevents the hood from blocking the field of view of the lens and producing vignetting. Flower shaped lens hoods are most often used on zoom lenses as a normal lens hood may block the field of view on some zoom settings.

Lens hoods are more prominent in telephoto lenses because the field of view has a smaller viewing angle than of wide-angle lenses. For wide angle lenses, the length of the hood (away from the end of the lens) cannot be as long as those for telephoto lenses because of the viewing angle.

Lens hoods are often designed to fit onto the matching lens facing either forward, for normal use, or backwards, so that the hood may be stored with the lens without occupying much additional space.

In addition, they offer some level of protection for the lens due to the hood extending farther than the lens itself.

Lens without and with hood

Varifocal Lens

2008-06-19T15:21:00.002+08:00

Many so-called "zoom" lenses, particularly in the case of fixed lens cameras, are actually varifocal lenses, which gives lens designers more flexibility in optical design trade-offs (focal length range, maximum aperture, size, weight, cost) than true parfocal zoom, and which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length ("zooming") making operation essentially the same as a true parfocal zoom.

Zoom Lens: Design

2008-06-19T15:10:00.010+08:00

A simple zoom lens system

There are many possible designs for zoom lenses, the most complex ones having upwards of thirty individual lens elements, and multiple moving parts. Most however follow the same basic design. Generally they consist of a number of individual lenses that may be either fixed, or slide axially along the body of the lens. As the magnification of a zoom lens changes, it is necessary to compensate for any movement of the focal plane to keep the focussed image sharp. This compensation may be done by mechanical means (moving the complete lens assembly as the magnification of the lens changes), or optically (arranging the position focal plane to vary as little as possible as the lens is zoomed).

A simple scheme for a zoom lens divides the assembly into two parts: a focussing lens similar to a standard, fixed-focal-length photographic lens, preceded by an afocal zoom system, an arrangement of fixed and movable lens elements that does not focus the light, but alters the size of a beam of light travelling through it, and thus the overall magnification of the lens system.

Movement of lenses in an afocal zoom system.

In this simple optically compensated zoom lens, the afocal system consists of two positive (converging) lenses of equal focal length (lenses L1 and L3) with a negative (diverging) lens (L2) between them, with an absolute focal length less than half that of the positive lenses. Lens L3 is fixed, but lenses L1 and L2 can be moved axially, and do so in a fixed, non-linear relationship. This movement is usually performed by a complex arrangement of gears and cams in the lens housing, although some modern zoom lenses use computer-controlled servos to perform this positioning.

As the negative lens L2 moves from the front to the back of the lens, the lens L1 moves forward and then backward in a parabolic arc. In doing so, the overall angular magnification of the system varies, changing the effective focal length of the complete zoom lens.

At each of the three points shown, the three-lens system is afocal (neither diverging or converging the light), and so does not alter the position of the focal plane of the lens. Between these points, the system is not exactly afocal, but the variation in focal plane position can be very small (~±0.01 mm in a well-designed lens) and so this slight defocussing is not apparent.

An important issue in zoom lens design is the correction of optical aberrations (such as chromatic aberration, and in particular, field curvature) across the whole operating range of the lens; this is considerably harder in a zoom lens than a fixed lens, which need only correct the aberrations for one focal length. This problem was a major reason for the slow uptake of zoom lenses, with early designs being considerably inferior to contemporary fixed lenses, and usable only with a narrow range of f-numbers. Modern optical design techniques have enabled the construction of zoom lenses with good aberration correction over widely variable focal lengths and apertures.

Whereas lenses used in cinematography and video applications are required to maintain focus as the focal length is changed, there is no such requirement for still photography, or if a zoom lens is used as a projection lens. Since it is harder to construct a lens that does not change focus with the same image quality as one that does, the latter applications often have lenses that require refocussing once the focal length has changed (and thus strictly speaking are varifocal lenses, not zoom lenses). As most still cameras are autofocus these days, it hardly presents a problem.

Designers of zoom lenses with large zoom ratios often will trade one or more aberrations for higher image sharpness. For example, a greater degree of barrel distortion is tolerated in lenses that span the focal length range from wide angle to telephoto with a focal ratio of 10x or more than would be acceptable in a fixed focal length lens or a zoom lens with a lower ratio. Although modern design methods have been continually reducing this problem, barrel distortion of greater than one percent is common in these types of lenses. Another price paid is that at the extreme telephoto setting of the lens, the effective focal length changes significantly as the lens is focussed on nearer and nearer objects. The apparent focal length can more than halve as the lens is focussed from infinity to a few feet. To a lesser degree, this effect is also seen in fixed focal length lenses that move internal lens elements, rather than the entire lens, to effect changes in focal length.

Simplified zoom lens in operation

Zoom Lens: History

2008-06-19T15:03:00.002+08:00

Early forms of zoom lenses were used in optical telescopes to provide continuous variation of the magnification of the image, and this was first reported in the proceedings of the Royal Society in 1834.

Early patents for telephoto lenses also included movable lens elements which could be adjusted to change the overall focal length of the lens.

Lenses such as these are now called varifocal lenses, in that as the focal length is changed, the position of the focal plane also moves, requiring readjustment of the focusing of the lens after each change.

The first true zoom lens, which retained near-sharp focus while the effective focal length of the lens assembly was changed, was patented in 1902 by Clile C. Allen (U.S. Patent 696,788).

The first industrial production was the Bell and Howell Cooke "Varo" 40–120 mm lens for 35mm movie cameras introduced in 1932. The most impressive TV Zoom lens was the VAROTAL III from Rank Taylor Hobson from UK built in 1953. The Kilfitt 36–82 mm/2.8 Zoomar (refer photo) introduced in 1959 was the first zoom lens in regular production for still 35mm photography.

Since then, advances in optical design, particularly the use of computers for optical ray tracing, has made the design and construction of zoom lenses much easier, and they are now used widely in professional and amateur photography.

Zoom Lens: Applications

2008-06-19T14:58:00.002+08:00

Zoom lenses are often described by the ratio of their longest to shortest focal lengths. For example, a zoom lens with focal lengths ranging from 100 mm to 400 mm may be described as a 4:1 or "4×" zoom.

The term superzoom or hyperzoom is used to describe photographic zoom lenses with very large focal length factors, typically more than 4× and ranging up to 10× and even 14×. This ratio can be as high as 100× in professional television cameras. Currently, photographic zoom lenses beyond about 3× are not considered to have a quality on par with prime lenses, and constant fast aperture zooms (usually f/2.8 or f/2.0) are typically restricted to this range.

Photographic zoom lenses should not be confused with telephoto lenses, those with a narrow angle of view. Some zoom lenses are telephoto, some are wide-angle, and others cover a range from wide-angle to telephoto. Lenses in the latter group of zoom lenses, sometimes referred to as "normal" zooms, have displaced the fixed focal length lens as the popular one-lens selection on many contemporary cameras.

Some digital cameras allow cropping and enlarging of a captured image, in order to emulate the effect of a longer focal length zoom lens (narrower angle of view). This is commonly known as digital zoom and results in a lower quality image than optical zoom, as no optical resolution is gained.

Many digital cameras, such as the Canon PowerShot A720 IS have both, combining them by first using the optical, then the digital zoom. The optical zoom in this case can be calculated by dividing 34.8/5.8 as it is written on the lens tube of the camera, resulting in the zoom factor 6.

In addition to its photographic use, the afocal part of a zoom lens can be used as a telescope of variable magnification to make an adjustable beam expander. This can be used, for example, to change the size of a laser beam so that the irradiance of the beam can be varied.

Zoom Lens

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A zoom lens is a mechanical assembly of lens elements with the ability to vary its focal length (and thus angle of view), as opposed to a fixed focal length (FFL) lens. They are commonly used with still, video, motion picture cameras, projectors, some binoculars, microscopes, telescopes, and other optical instruments.

Nikkor 28-200 mm zoom lens,
extended to 200 mm at left
and collapsed to 28 mm
focal length at right

Chromatic or Color Aberration

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In optical systems composed of lenses, the position, magnitude and errors of the image depend upon the refractive indices of the glass employed (see Lens (optics), and above, Monochromatic Aberration). Since the index of refraction varies with the color or wavelength of the light (see dispersion), it follows that a system of lenses (uncorrected) projects images of different colors in somewhat different places and sizes and with different aberrations; i.e. there are chromatic differences of the distances of intersection, of magnifications, and of monochromatic aberrations.

If mixed light be employed (e.g. white light) all these images are formed; and since they are all ultimately intercepted by a plane (the retina of the eye, a focusing screen of a camera, etc.), they cause a confusion, named chromatic aberration; for instance, instead of a white margin on a dark background, there is perceived a colored margin, or narrow spectrum. The absence of this error is termed achromatism, and an optical system so corrected is termed achromatic. A system is said to be chromatically under-corrected when it shows the same kind of chromatic error as a thin positive lens, otherwise it is said to be over-corrected.

If, in the first place, monochromatic aberrations be neglected — in other words, the Gaussian theory be accepted — then every reproduction is determined by the positions of the focal planes, and the magnitude of the focal lengths, or if the focal lengths, as ordinarily happens, be equal, by three constants of reproduction. These constants are determined by the data of the system (radii, thicknesses, distances, indices, &c., of the lenses); therefore their dependence on the refractive index, and consequently on the color, are calculable (the formulae are given in Czapski-Eppenstein, Grundzuge der Theorie der optischen Instrumente (1903, p. 166).

The refractive indices for different wavelengths must be known for each kind of glass made use of. In this manner the conditions are maintained that any one constant of reproduction is equal for two different colors, i.e. this constant is achromatized. For example, it is possible, with one thick lens in air, to achromatize the position of a focal plane of the magnitude of the focal length. If all three constants of reproduction be achromatized, then the Gaussian image for all distances of objects is the same for the two colors, and the system is said to be in stable achromatism.

In practice it is more advantageous (after Abbe) to determine the chromatic aberration (for instance, that of the distance of intersection) for a fixed position of the object, and express it by a sum in which each component conlins the amount due to each refracting surface (see Czapski-Eppenstein, op. cit. p. 170; A. Konig in M. v. Rohr's collection, Die Bilderzeugung, p. 340).

In a plane containing the image point of one color, another colour produces a disk of confusion; this is similar to the confusion caused by two zones in spherical aberration. For infinitely distant objects the radius Of the chromatic disk of confusion is proportional to the linear aperture, and independent of the focal length (vide supra, Monochromatic Aberration of the Axis Point); and since this disk becomes the less harmful with an increasing image of a given object, or with increasing focal length, it follows that the deterioration of the image is proportional to the ratio of the aperture to the focal length, i.e. the relative aperture. (This explains the gigantic focal lengths in vogue before the discovery of achromatism.)

Examples:

(a) In a very thin lens, in air, only one constant of reproduction is to be observed, since the focal length and the distance of the focal point are equal. If the refractive index for one color be n, and for another n+dn, and the powers, or reciprocals of the focal lengths, be f and f + df, then (1) df/f = dn/(n-1) = 1/n; dn is called the dispersion, and n the dispersive power of the glass.

(b) Two thin lenses in contact: let f1 and f2 be the powers corresponding to the lenses of refractive indices n1 and n2 and radii r'1, r"1, and r'2, r"2 respectively; let f denote the total power, and df, dn1, dn2 the changes of f, n1, and n2 with the color. Then the following relations hold: —

(2) f = f1-f2== (n1 - 1)(1/r'1-1/r1) +(n2-1)(1/r'2 - 1/r2) = (n1 - 1)k1 + (n2 - 1)k2; and

(3) df = k1dn1 + k2dn2. For achromatism df = 0, hence, from (3),

(4) k1/k2 = -dn2 / dn1, or f1/f2 = -n1/n2. Therefore f1 and f2 must have different algebraic signs, or the system must be composed of a collective and a dispersive lens. Consequently the powers of the two must be different (in order that f be not zero (equation 2)), and the dispersive powers must also be different (according to 4).

Newton failed to perceive the existence of media of different dispersive powers required by achromatism; consequently he constructed large reflectors instead of refractors. James Gregory and Leonhard Euler arrived at the correct view from a false conception of the achromatism of the eye; this was determined by Chester More Hall in 1728, Klingenstierna in 1754 and by Dollond in 1757, who constructed the celebrated achromatic telescopes. (See telescope.)

Glass with weaker dispersive power (greater v) is named crown glass; that with greater dispersive power, flint glass. For the construction of an achromatic collective lens (f positive) it follows, by means of equation (4), that a collective lens I. of crown glass and a dispersive lens II. of flint glass must be chosen; the latter, although the weaker, corrects the other chromatically by its greater dispersive power. For an achromatic dispersive lens the converse must be adopted. This is, at the present day, the ordinary type, e.g., of telescope objective (fig. 10); the values of the four radii must satisfy the equations (2) and (4).

Two other conditions may also be postulated: one is always the elimination of the aberration on the axis; the second either the Herschel or Fraunhofer Condition, the latter being the best vide supra, Monochromatic Aberration). In practice, however, it is often more useful to avoid the second condition by making the lenses have contact, i.e. equal radii. According to P. Rudolph (Eder's Jahrb. f. Photog., 1891, 5, p. 225; 1893, 7, p. 221), cemented objectives of thin lenses permit the elimination of spherical aberration on the axis, if, as above, the collective lens has a smaller refractive index; on the other hand, they permit the elimination of astigmatism and curvature of the field, if the collective lens has a greater refractive index (this follows from the Petzval equation; see L. Seidel, Astr. Nachr., 1856, p. 289).

Should the cemented system be positive, then the more powerful lens must be positive; and, according to (4), to the greater power belongs the weaker dispersive power (greater v), that is to say, crown glass; consequently the crown glass must have the greater refractive index for astigmatic and plane images. In all earlier kinds of glass, however, the dispersive power increased with the refractive index; that is, v decreased as n increased; but some of the Jena glasses by E. Abbe and O. Schott were crown glasses of high refractive index, and achromatic systems from such crown glasses, with flint glasses of lower refractive index, are called the new achromats, and were employed by P. Rudolph in the first anastigmats (photographic objectives).

Instead of making df vanish, a certain value can be assigned to it which will produce, by the addition of the two lenses, any desired chromatic deviation, e.g. sufficient to eliminate one present in other parts of the system. If the lenses I. and II. be cemented and have the same refractive index for one color, then its effect for that one color is that of a lens of one piece; by such decomposition of a lens it can be made chromatic or achromatic at will, without altering its spherical effect. If its chromatic effect (df/f) be greater than that of the same lens, this being made of the more dispersive of the two glasses employed, it is termed hyper-chromatic.

For two thin lenses separated by a distance D the condition for achromatism is D = v1f1+v2f2; if v1=v2 (e.g. if the lenses be made of the same glass), this reduces to D= 1/2 (f1+f2), known as the condition for oculars.

If a constant of reproduction, for instance the focal length, be made equal for two colors, then it is not the same for other colors, if two different glasses are employed. For example, the condition for achromatism (4) for two thin lenses in contact is fulfilled in only one part of the spectrum, since dn2/dn1 varies within the spectrum. This fact was first ascertained by J. Fraunhofer, who defined the colors by means of the dark lines in the solar spectrum; and showed that the ratio of the dispersion of two glasses varied about 20% from the red to the violet (the variation for glass and water is about 50%). If, therefore, for two colors, a and b, fa = fb = f, then for a third color, c, the focal length is different; that is, if c lies between a and b, then fc

In fig. 11, taken from M. von Rohr's Theorie und Geschichte des photographischen Objectivs, the abscissae are focal lengths, and the ordinates wavelengths. The Fraunhofer lines used are shown in the table to the right of the figure.

A' - 767.7 nm

C - 656.3 nm

D - 589.3 nm

Green Hg. - 546.1 nm

F - 486.2 nm

G' - 454.1 nm

Violet Hg. - 405.1 nm

and the focal lengths are made equal for the lines C and F. In the neighborhood of 550 nm the tangent to the curve is parallel to the axis of wave-lengths; and the focal length varies least over a fairly large range of color, therefore in this neighborhood the color union is at its best.

Moreover, this region of the spectrum is that which appears brightest to the human eye, and consequently this curve of the secondary on spectrum, obtained by making fc = fF, is, according to the experiments of Sir G. G. Stokes (Proc. Roy. Soc., 1878), the most suitable for visual instruments (optical achromatism,). In a similar manner, for systems used in photography, the vertex of the color curve must be placed in the position of the maximum sensibility of the plates; this is generally supposed to be at G'; and to accomplish this the F and violet mercury lines are united.

This artifice is specially adopted in objectives for astronomical photography (pure actinic achromatism). For ordinary photography, however, there is this disadvantage: the image on the focusing-screen and the correct adjustment of the photographic sensitive plate are not in register; in astronomical photography this difference is constant, but in other kinds it depends on the distance of the objects. On this account the lines D and G' are united for ordinary photographic objectives; the optical as well as the actinic image is chromatically inferior, but both lie in the same place; and consequently the best correction lies in F (this is known as the actinic correction or freedom from chemical focus).

Should there be in two lenses in contact the same focal lengths for three colours a, b, and c, i.e. fa = fb = fc = f, then the relative partial dispersion (nc-nb) (na-nb) must be equal for the two kinds of glass employed. This follows by considering equation (4) for the two pairs of colors ac and bc. Until recently no glasses were known with a proportional degree of absorption; but R. Blair (Trans. Edin. Soc., 1791, 3, p. 3), P. Barlow, and F. S. Archer overcame the difficulty by constructing fluid lenses between glass walls.

Fraunhofer prepared glasses which reduced the secondary spectrum; but permanent success was only assured on the introduction of the Jena glasses by E. Abbe and O. Schott. In using glasses not having proportional dispersion, the deviation of a third colour can be eliminated by two lenses, if an interval be allowed between them; or by three lenses in contact, which may not all consist of the old glasses. In uniting three colors an achromatism of a higher order is derived; there is yet a residual tertiary spectrum, but it can always be neglected.

The Gaussian theory is only an approximation; monochromatic or spherical aberrations still occur, which will be different for different colors; and should they be compensated for one color, the image of another color would prove disturbing. The most important is the chromatic difference of aberration of the axis point, which is still present to disturb the image, after par-axial rays of different colors are united by an appropriate combination of glasses. If a collective system be corrected for the axis point for a definite wave-length, then, on account of the greater dispersion in the negative components — the flint glasses, — over-correction will arise for the shorter wavelengths (this being the error of the negative components), and under-correction for the longer wave-lengths (the error of crown glass lenses preponderating in the red).

This error was treated by Jean le Rond d'Alembert, and, in special detail, by C. F. Gauss. It increases rapidly with the aperture, and is more important with medium apertures than the secondary spectrum of par-axial rays; consequently, spherical aberration must be eliminated for two colors, and if this be impossible, then it must be eliminated for those particular wave-lengths which are most effectual for the instrument in question (a graphical representation of this error is given in M. von Rohr, Theorie und Geschichte des photographischen Objectivs).

The condition for the reproduction of a surface element in the place of a sharply reproduced point — the constant of the sine relationship must also be fulfilled with large apertures for several colors. E. Abbe succeeded in computing microscope objectives free from error of the axis point and satisfying the sine condition for several colors, which therefore, according to his definition, were aplanatic for several colors; such systems he termed apochromatic. While, however, the magnification of the individual zones is the same, it is not the same for red as for blue; and there is a chromatic difference of magnification.

This is produced in the same amount, but in the opposite sense, by the oculars, which Abbe used with these objectives (compensating oculars), so that it is eliminated in the image of the whole microscope. The best telescope objectives, and photographic objectives intended for three-color work, are also apochromatic, even if they do not possess quite the same quality of correction as microscope objectives do. The chromatic differences of other errors of reproduction have seldom practical importances.

Practical Elimination of Aberrations

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The classical imaging problem is to reproduce perfectly a finite plane (the object) onto another plane (the image) through a finite aperture. It is impossible to do so perfectly for more than one such pairs of planes (this was proven with increasing generality by Maxwell in 1858, by Bruns in 1895, and by Carathéodory in 1926, see summary in Walther, A., J. Opt. Soc. Am. A 6, 415–422 (1989)). For a single pair of planes (e.g. for a single focus setting of an objective), however, the problem could in principle be solved perfectly.

Practical methods solve this problem with an accuracy which mostly suffices for the special purpose of each species of instrument. The problem of finding a system which reproduces a given object upon a given plane with given magnification (insofar as aberrations must be taken into account) could be dealt with by means of the approximation theory; in most cases, however, the analytical difficulties were too great for older calculation methods but may be ameliorated by application of modern computer systems. Solutions, however, have been obtained in special cases (see A. Konig in M. von Rohr's Die Bilderzeugung, p. 373; K. Schwarzschild, Gottingen. Akad. Abhandl., 1905, 4, Nos. 2 and 3).

At the present time constructors almost always employ the inverse method: they compose a system from certain, often quite personal experiences, and test, by the trigonometrical calculation of the paths of several rays, whether the system gives the desired reproduction (examples are given in A. Gleichen, Lehrbuch der geometrischen Optik, Leipzig and Berlin, 1902). The radii, thicknesses and distances are continually altered until the errors of the image become sufficiently small. By this method only certain errors of reproduction are investigated, especially individual members, or all, of those named above. The analytical approximation theory is often employed provisionally, since its accuracy does not generally suffice.

In order to render spherical aberration and the deviation from the sine condition small throughout the whole aperture, there is given to a ray with a finite angle of aperture u* (width infinitely distant objects: with a finite height of incidence h*) the same distance of intersection, and the same sine ratio as to one neighboring the axis (u* or h* may not be much smaller than the largest aperture U or H to be used in the system). The rays with an angle of aperture smaller than u* would not have the same distance of intersection and the same sine ratio; these deviations are called zones, and the constructor endeavors to reduce these to a minimum.

The same holds for the errors depending upon the angle of the field of view, w: astigmatism, curvature of field and distortion are eliminated for a definite value, w*, zones of astigmatism, curvature of field and distortion, attend smaller values of w. The practical optician names such systems: corrected for the angle of aperture u* (the height of incidence h*) or the angle of field of view w*. Spherical aberration and changes of the sine ratios are often represented graphically as functions of the aperture, in the same way as the deviations of two astigmatic image surfaces of the image plane of the axis point are represented as functions of the angles of the field of view.

The final form of a practical system consequently rests on compromise; enlargement of the aperture results in a diminution of the available field of view, and vice versa. But the larger aperture will give the larger resolution. The following may be regarded as typical:

(1) Largest aperture; necessary corrections are — for the axis point, and sine condition; errors of the field of view are almost disregarded; example — high-power microscope objectives.

(2) Largest field of view; necessary corrections are — for astigmatism, curvature of field and distortion; errors of the aperture only slightly regarded; examples — photographic widest angle objectives and oculars.

Between these extreme examples stands the ordinary photographic objective: the portrait objective is corrected more with regard to aperture; objectives for groups more with regard to the field of view.

(3) Telescope objectives have small fields of view and aberrations on axis

are very important. Therefore zones will be kept as small as possible and design should emphasize simplicity. Because of this these lenses are the best for analytical computation.

Analytic Treatment of Aberrations

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The preceding review of the several errors of reproduction belongs to the Abbe theory of aberrations, in which definite aberrations are discussed separately; it is well suited to practical needs, for in the construction of an optical instrument certain errors are sought to be eliminated, the selection of which is justified by experience. In the mathematical sense, however, this selection is arbitrary; the reproduction of a finite object with a finite aperture entails, in all probability, an infinite number of aberrations. This number is only finite if the object and aperture are assumed to be infinitely small of a certain order; and with each order of infinite smallness, i.e. with each degree of approximation to reality (to finite objects and apertures), a certain number of aberrations is associated. This connection is only supplied by theories which treat aberrations generally and analytically by means of indefinite series.

A ray proceeding from an object point O (fig. 9) can be defined by the coordinates (ξ, η). Of this point O in an object plane I, at right angles to the axis, and two other coordinates (x, y), the point in which the ray intersects the entrance pupil, i.e. the plane II. Similarly the corresponding image ray may be defined by the points (ξ', η'), and (x', y'), in the planes I' and II'. The origins of these four plane coordinate systems may be collinear with the axis of the optical system; and the corresponding axes may be parallel. Each of the four coordinates ξ', η', x', y' are functions of ξ, η, x, y; and if it be assumed that the field of view and the aperture be infinitely small, then ξ, η, x, y are of the same order of infinitesimals; consequently by expanding ξ', η', x', y' in ascending powers of ξ, η, x, y, series are obtained in which it is only necessary to consider the lowest powers. It is readily seen that if the optical system be symmetrical, the origins of the coordinate systems collinear with the optical axis and the corresponding axes parallel, then by changing the signs of ξ, η, x, y, the values ξ', η', x', y' must likewise change their sign, but retain their arithmetical values; this means that the series are restricted to odd powers of the unmarked variables.

The nature of the reproduction consists in the rays proceeding from a point O being united in another point O'; in general, this will not be the case, for ξ', η' vary if ξ, η be constant, but x, y variable. It may be assumed that the planes I' and II' are drawn where the images of the planes I and II are formed by rays near the axis by the ordinary Gaussian rules; and by an extension of these rules, not, however, corresponding to reality, the Gauss image point O'₀, with coordinates ξ'₀, η'₀, of the point O at some distance from the axis could be constructed. Writing Dξ'=ξ'-ξ'₀ and Dη'=η'-η'₀, then Dξ' and Dη' are the aberrations belonging to ξ, η and x, y, and are functions of these magnitudes which, when expanded in series, contain only odd powers, for the same reasons as given above.

On account of the aberrations of all rays which pass through O, a patch of light, depending in size on the lowest powers of ξ, η, x, y which the aberrations contain, will be formed in the plane I'. These degrees, named by (J. Petzval (Bericht uber die Ergebnisse einiger dioptrischer Untersuchungen, Buda Pesth, 1843; Akad. Sitzber., Wien, 1857, vols. xxiv. xxvi.) the numerical orders of the image, are consequently only odd powers; the condition for the formation of an image of the mth order is that in the series for Dξ' and Dη' the coefficients of the powers of the 3rd, 5th…(m-2)th degrees must vanish. The images of the Gauss theory being of the third order, the next problem is to obtain an image of 5th order, or to make the coefficients of the powers of 3rd degree zero. This necessitates the satisfying of five equations; in other words, there are five alterations of the 3rd order, the vanishing of which produces an image of the 5th order.

Distortion of The Image

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If now the image is sufficiently sharp, inasmuch as the rays proceeding from every object point meet in an image point of satisfactory exactitude, it may happen that the image is distorted, i.e. not sufficiently like the object.

This error consists in the different parts of the object being reproduced with different magnifications; for instance, the inner parts may differ in greater magnification than the outer (barrel-shaped distortion), or conversely (cushion-shaped distortion). This effect is called lens distortion or image distortion, and there are algorithms to correct it.

Systems free of distortion are called orthoscopic (orthos, right, skopein to look) or rectilinear (straight lines).

This aberration is quite distinct from that of the sharpness of reproduction; in unsharp, reproduction, the question of distortion arises if only parts of the object can be recognized in the figure.

If, in an unsharp image, a patch of light corresponds to an object point, the center of gravity of the patch may be regarded as the image point, this being the point where the plane receiving the image, e.g. a focusing screen, intersects the ray passing through the middle of the stop.

This assumption is justified if a poor image on the focusing screen remains stationary when the aperture is diminished; in practice, this generally occurs.

This ray, named by Abbe a principal ray (not to be confused with the principal rays of the Gaussian theory), passes through the center of the entrance pupil before the first refraction, and the center of the exit pupil after the last refraction. From this it follows that correctness of drawing depends solely upon the principal rays; and is independent of the sharpness or curvature of the image field.

Referring to fig. 8, we have O'Q'/OQ = a' tan w'/a tan w = 1/N, where N is the scale or magnification of the image.

For N to be constant for all values of w, a' tan w'/a tan w must also be constant. If the ratio a'/a be sufficiently constant, as is often the case, the above relation reduces to the condition of Airy, i.e. tan w'/ tan w= a constant.

This simple relation is fulfilled in all systems which are symmetrical with respect to their diaphragm (briefly named symmetrical or holosymmetrical objectives), or which consist of two like, but different-sized, components, placed from the diaphragm in the ratio of their size, and presenting the same curvature to it (hemisymmetrical objectives); in these systems tan w' / tan w = 1.

Curvature of The Field of The Image

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If the above errors be eliminated, the two astigmatic surfaces united, and a sharp image obtained with a wide aperture — there remains the necessity to correct the curvature of the image surface, especially when the image is to be received upon a plane surface, e.g. in photography. In most cases the surface is concave towards the system.

Aberration of Lateral Object Points With Broad Pencils. Coma.

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By opening the stop wider, similar deviations arise for lateral points as have been already discussed for axial points; but in this case they are much more complicated.

The course of the rays in the meridional section is no longer symmetrical to the principal ray of the pencil; and on an intercepting plane there appears, instead of a luminous point, a patch of light, not symmetrical about a point, and often exhibiting a resemblance to a comet having its tail directed towards or away from the axis.

From this appearance it takes its name. The unsymmetrical form of the meridional pencil—formerly the only one considered—is coma in the narrower sense only; other errors of coma have been treated by A. Konig and M. von Rohr (op. cit.), and later by A. Gullstrand (op. cit.; Ann. d. Phys., 1905, 18, p. 941).

Aberration of Lateral Object Points (Points Beyond The Axis) With Narrow Pencils. Astigmatism.

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A point O (fig. 6) at a finite distance from the, axis (or with an infinitely distant object, a point which subtends a finite angle at the system) is, in general, even then not sharply reproduced, if the pencil of rays issuing from it and traversing the system is made infinitely narrow by reducing the aperture stop; such a pencil consists of the rays which can pass from the object point through the now infinitely small entrance pupil.

It is seen (ignoring exceptional cases) that the pencil does not meet the refracting or reflecting surface at right angles; therefore it is astigmatic (Gr. a-, privative, stigmia, a point). Naming the central ray passing through the entrance pupil the axis of the pencil or principal ray, it can be said: the rays of the pencil intersect, not in one point, but in two focal lines, which can be assumed to be at right angles to the principal ray; of these, one lies in the plane containing the principal ray and the axis of the system, i.e. in the first principal section or meridional section, and the other at right angles to it, i.e. in the second principal section or sagittal section.

We receive, therefore, in no single intercepting plane behind the system, as, for example, a focusing screen, an image of the object point; on the other hand, in each of two planes lines O' and O" are separately formed (in neighboring planes ellipses are formed), and in a plane between O' and O" a circle of least confusion.

The interval O'O", termed the astigmatic difference, increases, in general, with the angle W made by the principal ray OP with the axis of the system, i.e. with the field of view. Two astigmatic image surfaces correspond to one object plane; and these are in contact at the axis point; on the one lie the focal lines of the first kind, on the other those of the second. Systems in which the two astigmatic surfaces coincide are termed anastigmatic or stigmatic.

Aberration of Elements, i.e. Smallest Objects at Right Angles To The Axis

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If rays issuing from O (fig. 5) be concurrent, it does not follow that points in a portion of a plane perpendicular at O to the axis will be also concurrent, even if the part of the plane be very small.

With a considerable aperture, the neighboring point N will be reproduced, but attended by aberrations comparable in magnitude to ON. These aberrations are avoided if, according to Abbe, the sine condition, sin u'1/sin u1=sin u'2/sin u2, holds for all rays reproducing the point O.

If the object point O is infinitely distant, u1 and u2 are to be replaced by pi and h2, the perpendicular heights of incidence; the sine condition then becomes sin u'1/h1 sin u'2/h2.

A system fulfilling this condition and free from spherical aberration is called aplanatic (Greek a-, privative, plann, a wandering).

This word was first used by Robert Blair (d. 1828), professor of practical astronomy at Edinburgh University, to characterize a superior achromatism, and, subsequently, by many writers to denote freedom from spherical aberration.

Both the aberration of axis points, and the deviation from the sine condition, rapidly increase in most (uncorrected) systems with the aperture.

Aberration of Axial Points (Spherical Aberration In The Restricted Sense)

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Let S (fig.5) be any optical system, rays proceeding from an axis point O under an angle u1 will unite in the axis point O'1; and those under an angle u2 in the axis point O'2. If there is refraction at a collective spherical surface, or through a thin positive lens, O'2 will lie in front of O'1 so long as the angle u2 is greater than u1 (under correction); and conversely with a dispersive surface or lenses (over correction).

The caustic, in the first case, resembles the sign > (greater than); in the second < (less than). If the angle u1 is very small, O'1 is the Gaussian image; and O'1 O'2 is termed the longitudinal aberration, and O'1R the lateral aberration of the pencils with aperture u2. If the pencil with the angle u2 is that of the maximum aberration of all the pencils transmitted, then in a plane perpendicular to the axis at O'1 there is a circular disk of confusion of radius O'1R, and in a parallel plane at O'2 another one of radius O'2R2; between these two is situated the disk of least confusion.

The largest opening of the pencils, which take part in the reproduction of O, i.e. the angle u, is generally determined by the margin of one of the lenses or by a hole in a thin plate placed between, before, or behind the lenses of the system. This hole is termed the stop or diaphragm; Abbe used the term aperture stop for both the hole and the limiting margin of the lens. The component S1 of the system, situated between the aperture stop and the object O, projects an image of the diaphragm, termed by Abbe the entrance pupil; the exit pupil is the image formed by the component S2, which is placed behind the aperture stop. All rays which issue from O and pass through the aperture stop also pass through the entrance and exit pupils, since these are images of the aperture stop.

Since the maximum aperture of the pencils issuing from O is the angle u subtended by the entrance pupil at this point, the magnitude of the aberration will be determined by the position and diameter of the entrance pupil. If the system be entirely behind the aperture stop, then this is itself the entrance pupil (front stop); if entirely in front, it is the exit pupil (back stop).

If the object point be infinitely distant, all rays received by the first member of the system are parallel, and their intersections, after traversing the system, vary according to their perpendicular height of incidence, i.e. their distance from the axis. This distance replaces the angle u in the preceding considerations; and the aperture, i.e. the radius of the entrance pupil, is its maximum value.

Monochromatic Aberration

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The elementary theory of optical systems leads to the theorem: Rays of light proceeding from any object point unite in an image point; and therefore an object space is reproduced in an image space.

The introduction of simple auxiliary terms, due to C. F. Gauss (Dioptrische Untersuchungen, Göttingen, 1841), named the focal lengths and focal planes, permits the determination of the image of any object for any system (see lens).

The Gaussian theory, however, is only true so long as the angles made by all rays with the optical axis (the symmetrical axis of the system) are infinitely small, i.e. with infinitesimal objects, images and lenses; in practice these conditions are not realized, and the images projected by uncorrected systems are, in general, ill defined and often completely blurred, if the aperture or field of view exceeds certain limits.

The investigations of James Clerk Maxwell (Phil.Mag., 1856; Quart. Journ. Math., 1858, and Ernst Abbe) showed that the properties of these reproductions, i.e. the relative position and magnitude of the images, are not special properties of optical systems, but necessary consequences of the supposition (in Abbe) of the reproduction of all points of a space in image points (Maxwell assumes a less general hypothesis), and are independent of the manner in which the reproduction is effected.

These authors proved, however, that no optical system can justify these suppositions, since they are contradictory to the fundamental laws of reflexion and refraction. Consequently the Gaussian theory only supplies a convenient method of approximating to reality; and no constructor would attempt to realize this unattainable ideal.

All that at present can be attempted is, to reproduce a single plane in another plane; but even this has not been altogether satisfactorily accomplished, aberrations always occur, and it is improbable that these will ever be entirely corrected.

This, and related general questions, have been treated — besides the above-mentioned authors — by M. Thiesen (Berlin. Akad. Sitzber., 1890, xxxv. 799; Berlin. Phys. Ges. Verh., 1892) and H. Bruns (Leipzig. Math. Phys. Ber., 1895, xxi. 325) by means of Sir W. R. Hamilton's characteristic function (Irish Acad. Trans., Theory of Systems of Rays, 1828, et seq.). Reference may also be made to the treatise of Czapski-Eppenstein, pp. 155-161.

A review of the simplest cases of aberration will now be given.

Optical Aberration

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Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system.

It occurs when light from one point of an object after transmission through the system does not converge into (or does not diverge from) a single point. Instrument-makers need to correct optical systems to compensate for aberration.

The articles on reflection, refraction and caustics discuss the general features of reflected and refracted rays.

Aberrations fall into two classes:

Monochromatic aberrations (Gr. monos, one) produced without dispersion. These include the aberrations at reflecting surfaces of any colored light, and at refracting surfaces of monochromatic light of single wavelength. These include:
- Piston
- Tilt
- Defocus
- Spherical
- Coma
- Astigmatism
- Curvature of field
- Image distortion
Chromatic aberrations (Gr. croma, color), where a system disperses the various wavelengths of light
- Axial, or longitudinal, chromatic aberration
- Lateral, or transverse, chromatic aberration

Piston and tilt are not actually true optical aberrations, as they do not represent or model curvature in the wavefront. If an otherwise perfect wavefront is "aberrated" by piston and tilt, it will still form a perfect, aberration-free image, only shifted to a different position. Defocus is the lowest order true optical aberration.

Aperture & Focal Length

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The two main optical parameters of a photographic lens are the maximum aperture and the focal length. The focal length determines the angle of view, and the size of the image relative to that of the object, while the maximum aperture limits the brightness of the image and the fastest shutter speed usable. A popular third consideration is close focusing distance.

The maximum usable aperture of a lens is usually specified as the focal ratio or f-number, which is equal to the focal length divided by the effective aperture (or entrance pupil) diameter in the same units. The lower the number, the more light per unit area is delivered to the focal plane. Larger apertures (smaller f-numbers) provide a much shallower depth of field than smaller apertures, other conditions being equal. Practical lens assemblies may also contain mechanisms to deal with measuring light, secondary apertures for flare reduction, and mechanisms to hold the aperture open until the instant of exposure to allow SLR cameras to focus with a brighter image with shallower depth of field, theoretically allowing better focus accuracy.

Focal lengths are usually specified in millimetres (mm), but older lenses marked in centimetres (cm) and inches are still to be found. For a given film or sensor size, specified by the length of the diagonal, a lens may be classified as

Normal lens: angle of view of the diagonal about 50° and a focal length approximately equal to the diagonal produces this angle.
Macro lens: angle of view narrower than 25° and focal length longer than normal. These lenses are used for close-ups, e.g., for images of the same size as the object. They usually feature a flat field as well, which means that the subject plane is exactly parallel with the film plane.
Wide-angle lens: angle of view wider than 60° and focal length shorter than normal.
Telephoto lens or long-focus lens: angle of view narrower and focal length longer than normal. A distinction is sometimes made between a long-focus lens and a true telephoto lens: the telephoto lens uses a telephoto group to be physically shorter than its focal length.

One of Canon's most popular wide-angle
zoom lenses, the 17–40mm f/4 L.

The Canon 85mm f/1.8 is a compact lens popular
with portrait photographers. Its large aperture
can be used to minimize flash requirements
or to produce a shallow depth of field.

The 35mm film format is so prevalent that a 90mm lens, for example, is sometimes assumed to be a moderate telephoto; but for the 7×5cm format it is normal, while on the large 5×4 inch format it is a wide-angle. In general, the smaller the film or sensor surface, the smaller the angle of view. This can be corrected with lenses with shorter focal lengths.

An example of how lens choice affects angle of view. The photos below were taken
by a 35 mm camera at a constant distance from the subject.

28mm lens

50mm lens

70mm lens

210mm lens

A side effect of using lenses of different focal lengths is the different distances from which a subject can be framed, resulting in a different perspective. Photographs can be taken of a person stretching out a hand with a wideangle, a normal lens, and a telephoto, which contain exactly the same image size by changing the distance from the subject. But the perspective will be different. With the wideangle, the hands will be exaggeratedly large relative to the head. As the focal length increases, the emphasis on the outstretched hand decreases.

However, if pictures are taken from the same distance, and enlarged and cropped to contain the same view, the pictures will have identical perspective. A moderate long-focus (telephoto) lens is often recommended for portraiture because the perspective corresponding to the longer shooting distance is considered to look more flattering.

Photographic Lens

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A photographic lens (also known as objective lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.

While in principle a simple convex lens will suffice, in practice a compound lens made up of a number of optical lens elements is required to correct (as much as possible) the many optical aberrations that arise. Some aberrations will be present in any lens system. It is the job of the lens designer to balance these out and produce a design that is suitable for photographic use and possibly mass production.

There is no major difference in principle between a lens used for a camera, a telescope, a microscope, or other apparatus, but the detailed design and construction are different.

A lens may be permanently fixed to a camera, or it may be interchangeable with lenses of different focal lengths, apertures, and other properties.

Light Meter - Use In Illumination

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Light meters or light detectors are also used in illumination. Their purpose is to measure the illumination level in the interior and to switch off or reduce the output level of luminaires. This can greatly reduce the energy burden of the building by significantly increasing the efficiency of its lighting system. It is known that 20 to 60 percent of all electrical power in a building is consumed by illumination.

It is therefore recommended to use light meters in lighting systems, especially in rooms where one cannot expect users to pay attention to manually switching off the lights. Examples include hallways, stairs, and big halls.

There are, however, significant obstacles to overcome in order to achieve a successful implementation of light meters in lighting systems, of which user acceptance is by far the most formidable. Unexpected or too frequent switching and too bright or too dark rooms are very annoying and disturbing for users of the rooms. Therefore, different switching algorithms have been developed:

difference algorithm, where light switch on lower light level than they switch off, thus taking care that the difference between the light level of the 'on' state and 'off' state is not too big
time delay algorithms:
- certain amount of time must pass since the last switch
- certain amount of time of sufficient illumination.

Exposure Determination With A Neutral Test Card

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If a scene differs considerably from a statistically average scene, a wide-angle averaging reflected-light measurement may not indicate the correct exposure.

To simulate an average scene, a substitute measurement sometimes is made of a neutral test card, or gray card.

At best, a flat card is an approximation to a three-dimensional scene, and measurement of a test card may lead to underexposure unless adjustment is made.

The instructions for a Kodak neutral test card recommend that the indicated exposure be increased by ½ step for a frontlighted scene in sunlight.

The instructions also recommend that the test card be held vertically and faced in a direction midway between the Sun and the camera; similar directions are also given in the Kodak Professional Photoguide.

The combination of exposure increase and the card orientation gives recommended exposures that are reasonably close to those given by an incident-light meter with a hemispherical receptor when metering with an off-axis light source.

In practice, additional complications may arise.

Many neutral test cards are far from perfectly diffuse reflectors, and specular reflections can cause increased reflected-light meter readings that, if followed, would result in underexposure.

It is possible that the neutral test card instructions include a correction for specular reflections.

Calibrated Reflectance

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It is commonly stated that reflected-light meters are calibrated to an 18% reflectance, but the calibration has nothing to do with reflectance, as should be evident from the exposure formulas. However, some notion of reflectance is implied by a comparison of incident- and reflected-light meter calibration.

Combining the reflected-light and incident-light exposure equations and rearranging gives

Reflectance

ζ

is defined as

A uniform perfect diffuser (i.e., one following Lambert's cosine law) of luminance

L

emits a flux density of

π

L

; reflectance then is

Illuminance is measured with a flat receptor. It is straightforward to compare an incident-light measurement using a flat receptor with a reflected-light measurement of a uniformly illuminated flat surface of constant reflectance. Using values of 12.5 for

K

and 250 for

C

gives

With a K of 14, the reflectance would be 17.6%, close to that of a standard 18% neutral test card. In theory, an incident-light measurement should agree with a reflected-light measurement of a test card of suitable reflectance that is perpendicular to the direction to the meter. However, a test card seldom is a uniform diffuser, so incident- and reflected-light measurements might differ slightly.

In a typical scene, many elements are not flat and are at various orientations to the camera, so that for practical photography, a hemispherical receptor usually has proven more effective for determining exposure. Using values of 12.5 for K and 330 for C gives

With a slightly revised definition of reflectance, this result can be taken as indicating that the average scene reflectance is approximately 12%. A typical scene includes shaded areas as well as areas that receive direct illumination, and a wide-angle averaging reflected-light meter responds to these differences in illumination as well as differing reflectances of various scene elements. Average scene reflectance then would be

where “effective scene illuminance” is that measured by a meter with a hemispherical receptor.

ISO 2720:1974 calls for reflected-light calibration to be measured by aiming the receptor at a transilluminated diffuse surface, and for incident-light calibration to be measured by aiming the receptor at a point source in a darkened room.

For a perfectly diffusing test card and perfectly diffusing flat receptor, the comparison between a reflected-light measurement and an incident-light measurement is valid for any position of the light source. However, the response of a hemispherical receptor to an off-axis light source is approximately that of a cardioid rather than a cosine, so the 12% “reflectance” determined for an incident-light meter with a hemispherical receptor is valid only when the light source is on the receptor axis.

Exposure Meter Calibration

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In most cases, an incident-light meter will cause a medium tone to be recorded as a medium tone, and a reflected-light meter will cause whatever is metered to be recorded as a medium tone. What constitutes a “medium tone” depends on meter calibration and several other factors, including film processing or digital image conversion.

Meter calibration establishes the relationship between subject lighting and recommended camera settings. The calibration of photographic light meters is covered by ISO 2720:1974.

Exposure equations
For reflected-light meters, camera settings are related to ISO speed and subject luminance by the reflected-light exposure equation:

where

$N$ is the relative aperture (f-number)
$t$ is the exposure time ("shutter speed")
$L$ is the average scene luminance
$S$ is the ISO linear speed
$K$ is the reflected-light meter calibration constant

For incident-light meters, camera settings are related to ISO speed and subject illuminance by the incident-light exposure equation:

where

$E$ is the illuminance
$C$ is the incident-light meter calibration constant

Calibration constants
Determination of calibration constants has been largely subjective; ISO 2720:1974 states that

The constants K and C shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a large number of observers, of a number of photographs, for which the exposure was known, obtained under various conditions of subject manner and over a range of luminances.

In practice, the variation of the calibration constants among manufacturers is considerably less than this statement might imply, and values have changed little since the early 1970s.

ISO 2720:1974 recommends a range for K of 10.6 to 13.4 with luminance in cd/m². Two values for K are in common use: 12.5 (Canon, Nikon, and Sekonic[1]) and 14 (Kenko[2] and Pentax); the difference between the two values is approximately 1/6 EV.

The earliest calibration standards were developed for use with wide-angle averaging reflected-light meters (Jones and Condit 1941). Although wide-angle average metering has largely given way to other metering sensitivity patterns (e.g., spot, center-weighted, and multi-segment), the values for K determined for wide-angle averaging meters have remained.

The incident-light calibration constant depends on the type of light receptor. Two receptor types are common: flat (cosine-responding) and hemispherical (cardioid-responding). With a flat receptor, ISO 2720:1974 recommends a range for C of 240 to 400 with illuminance in lux; a value of 250 is commonly used. A flat receptor typically is used for measurement of lighting ratios, for measurement of illuminance, and occasionally, for determining exposure for a flat subject.

For determining practical photographic exposure, a hemispherical receptor has proven more effective. Don Norwood, inventor of incident-light exposure meter with a hemispherical receptor, thought that a sphere was a reasonable representation of a photographic subject. According to his patent (Norwood 1938), the objective was

to provide an exposure meter which is substantially uniformly responsive to light incident upon the photographic subject from practically all directions which would result in the reflection of light to the camera or other photographic register.

and the meter provided for "measurement of the effective illumination obtaining at the position of the subject."

With a hemispherical receptor, ISO 2720:1974 recommends a range for C of 320 to 540 with illuminance in lux; in practice, values typically are between 320 (Minolta) and 340 (Sekonic). The relative responses of flat and hemispherical receptors depend upon the number and type of light sources; when each receptor is pointed at a small light source, a hemispherical receptor with C = 330 will indicate an exposure approximately 0.40 step greater than that indicated by a flat receptor with C = 250. With a slightly revised definition of illuminance, measurements with a hemispherical receptor indicate “effective scene illuminance.”

Light Meter - Use In Photography

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A handheld digital ambient light meter,

showing an f-stop of 5.6 for 24 frame/s
500 ISO filming

The earliest type of light meters were called extinction meters and contained a numbered or lettered row of neutral density filters of increasing density. The photographer would position the meter in front of their subject and note the filter with the greatest density that still allowed incident light to pass through. The letter or number corresponding to the filter was used as an index into a chart of appropriate aperture and shutter speed combinations for a given film speed.

Extinction meters suffered from the problem that they depended on the light sensitivity of the human eye (which can vary from person to person) and subjective interpretation.

Later meters removed the human element and relied on technologies incorporating selenium, CdS, and silicon photodetectors.

Selenium and silicon light meters use sensors that are photovoltaic. These sensors generate a voltage proportional to light exposure. Selenium sensors generate enough voltage for direct connection to a meter. Silicon sensors need an amplification circuit and require a power source such as batteries to operate. CdS light meters use a sensor based on photoresistance. These also require a battery to operate. Most modern light meters use silicon or CdS sensors. They indicate the exposure either with a needle galvanometer or on an LCD screen.

Many modern consumer still and video cameras include a built-in meter that measures a scene-wide light level and are able to make an approximate measure of appropriate exposure based on that. Photographers working with controlled lighting and cinematographers use handheld light meters to precisely measure the light falling on various parts of their subjects and use suitable lighting to produce the desired exposure levels.

There are two general types of light meters: reflected-light and incident-light. Reflected-light meters measure the light reflected by the scene to be photographed. All in-camera meters are reflected-light meters. Reflected-light meters are calibrated to show the appropriate exposure for “average” scenes. An unusual scene with a preponderance of light colors or specular highlights would have a higher reflectance; a reflected-light meter taking a reading would incorrectly compensate for the difference in reflectance and lead to underexposure.

This pitfall is avoided by incident-light meters which measure the amount of light falling on the subject using an integrating sphere (usually, a translucent hemispherical plastic dome is used to approximate this). Because the incident-light reading is independent of the subject's reflectance, it is less likely to lead to incorrect exposures for subjects with unusual average reflectance. Taking an incident-light reading requires placing the meter at the subject's position and pointing it in the general direction of the camera, something not always achievable in practice, e.g., in landscape photography where the subject is at infinity.

Another way to avoid under- or over-exposure for subjects with unusual reflectance is to use a spot meter: a reflected-light meter that measures light in a very tight cone, typically with a one degree angle. An experienced photographer can take multiple readings over the shadows, midrange and highlights of the scene to determine optimal exposure, using systems like the Zone System. Many modern cameras include sophisticated multi-segment metering systems that measure the luminance of different parts of the scene to determine the optimal exposure.

When using a film whose spectral sensitivity is not a good match to that of the light meter, for example orthochromatic black-and-white or infrared film, the meter may require special filters and re-calibration to match the sensitivity of the film.

There are other types of specialized photographic light meters. Flash meters are used in flash photography to verify correct exposure. Color meters are used where high fidelity in color reproduction is required. Densitometers are used in photographic reproduction.

Amateur analog light meter (1968, USSR)

An automatic light meter/exposure unit from
an 8 mm movie camera, based on a
galvanometer mechanism (center)
and a CdS photoresistor,
in opening at left.

Light Meter

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A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph.

Typically a light meter will include a computer, either digital or analogue, which allows the photographer to determine which shutter speed and f-number should be selected for an optimum exposure, given a certain lighting situation and film speed.

Light meters are also used in the fields of cinematography and scenic design, in order to determine the optimum light level for a scene.

They are used in the general field of lighting, where they can help to reduce the amount of waste light used in the home, light pollution outdoors, and plant growing to ensure proper light levels.

Passive Auto Focus (AF)

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Passive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system.

They generally do not direct any energy, such as ultrasonic sound or infrared light waves, toward the subject. (However, an autofocus assist beam of usually infrared light is required when there is not enough light to take passive measurements.)

Passive autofocusing can be achieved by phase detection or contrast measurement.

Phase detection is achieved by dividing the incoming light into pairs of images and comparing them. SIR TTL passive phase detection (secondary image registration, through the lens) is often used in film and digital SLR cameras.

The system uses a beam splitter (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera.

Two optical prisms capture the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simple range finder with a base identical to the lens' diameter.

The two images are then analysed for similar light intensity patterns (peaks and valleys) and the phase difference is calculated in order to find if the object is in front focus or back focus position. This instantly gives the exact direction of focusing and amount of focus ring's movement.

Although AF sensor is typically a one-dimensional photosensitive strip (only a few pixels high and a few dozen wide), some modern cameras (Canon EOS-1D, Nikon D2X) feature Area SIR sensors that are rectangular so as to provide two-dimensional intensity patterns.

Cross-type (CT) focus points have a pair of sensors oriented at 90° to one another, although one sensor typically requires a larger aperture to operate than the other.

Some cameras (Canon EOS-1D, Canon EOS 30D/40D) ) also have a few 'high precision' focus points with additional set of prisms and sensors; they are only active with 'fast lenses' of certain focal ratio.

Extended precision comes from the increased diameter of such lenses, so the base of the 'range finder' can be wider.

Phase detection system

Contrast measurement is achieved by measuring contrast within a sensor field, through the lens. The intensity difference between adjacent pixels of the sensor naturally increases with correct image focus. The optical system can thereby be adjusted until the maximum contrast is detected. In this method, AF does not involve actual distance measurement at all and is generally slower than phase detection systems, especially when operating under dim light. This is a common method in video cameras and consumer-level digital cameras that lack shutters and reflex mirrors. Some DSLRs (Olympus E-420, Panasonic L10, Nikon D300 in Tripod Mode) use this method when focusing in their live-view modes.

Active Auto Focus (AF)

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Active AF systems measure distance to the subject independently of the optical system, and subsequently adjust the optical system for correct focus.

There are various ways to measure distance, including ultrasonic sound waves and infrared light. In the first case, sound waves are emitted from the camera, and by measuring the delay in their reflection, distance to the subject is calculated.

Polaroid cameras including the Spectra and SX-70 were known for successfully applying this system. In the latter case, infrared light is usually used to triangulate the distance to the subject.

Compact cameras including the Nikon 35TiQD and 28TiQD, the Canon AF35M, and the Contax T2 and T3, as well as early video cameras, used this system.

An exception to the two-step approach is the mechanical autofocus provided in some enlargers, which adjust the lens directly.

Auto Focus (AF)

2008-06-18T16:04:00.002+08:00

Autofocus (or AF) is a feature of some optical systems that allows them to obtain (and in some systems to also continuously maintain) correct focus on a subject, instead of requiring the operator to adjust focus manually.

Focusing Screen

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Location of focusing screen (5) in an SLR camera

A focusing screen is a flat translucent material, usually ground glass, found in a system camera that allows the user of the camera to preview the framed image in a viewfinder.

Often, a focusing screen has etched markings that differ from model to model.

The history of the focusing screen is almost as long as the history of the camera. One could say that primitive cameras consisted of a box with a board holding the lens in the front and a focusing screen in the back that was replaced by the imaging medium (plate, film holder) before taking the picture.

The most common type of focusing screen in non-autofocus 35 mm SLR cameras is the split screen and microprism ring variation that aids focusing and became standard in the 1980s.

The microprism ring blurs the image unless the lens setting is in focus, the split screen shows part of the image split in two pieces. When both pieces are aligned the setting is in focus.

The drawback is that the prisms have a considerable light loss making low light focusing almost impossible.

Professional cameras give the photographer a choice of screens that are, depending on the camera model, more or less easy to replace. For low light situations the screen of choice is plain, for architectural images and very wide angle lenses the choice is one with a grid etched on it to control the perspective distortion, for fast focusing the split screen is the screen of choice and so on.

Cameras with interchangeable film formats (view cameras, field cameras and some medium format cameras) may have etchings on the focusing screen to show the limits of the films. Most of these cameras have either plain or grid screens because due to the size of the focusing screen the only focusing aid really needed is a magnifying glass.

Autofocus SLR cameras, both digital and film, usually have a plain screen. Some models have markings etched in them to denote the areas on which the camera focuses or calculates the exposure from.

Viewfinder

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Viewfinder is what the photographer looks through to compose, and in many cases to focus, the picture. Most viewfinders are separate, and suffer parallax, while the more sophisticated single-lens-reflex camera lets the viewfinder use the main optical system.

Viewfinders are used in many cameras of different types: still and movie, film, analog and digital. A zoom camera usually zooms its finder in synch with its lens.

Viewfinders can be optical or electronic. An optical viewfinder is simply a reversed telescope mounted to see what the camera will see. Its drawbacks are many, but it has one main advantage: it consumes no power. An electronic viewfinder is a CRT, LCD or OLED based display device.

In addition to its primary purpose, an electronic viewfinder can be used to replay previously captured material and usually has an on-screen-display. It is not uncommon for a camera to have two viewfinders. Here are two examples:

A digital still camera may have an optical viewfinder and an electronic one. The latter can be used to replay previously captured material, has an on-screen display, and can be switched off to save power.
A camcorder may have two viewfinders, both electronic. The first is viewed through a magnifying eyepiece. The second viewfinder is a mounted on the side of the camera and projected on a CRT or LCD-screen. Because it consumes more power, a method is often provided to turn it off to save energy.

Some special purpose cameras do not have viewfinders at all. These are, for example, web cameras and video surveillance cameras. They use external monitors as their viewfinders.

Self Timer

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A Self Timer is a device on a camera that, when enabled, gives a delay between the pressing of the shutter release and the shutter firing. It is most commonly used to allow photographers to take a photo of themselves, hence the name.

However, the self-timer mode is also often used to reduce camera shake when taking photographs in low light or with long telephoto lenses. The action of pressing the shutter release button shakes the camera to some degree. If the self-timer is used, the delay before the shutter fires allows the camera to be held more securely, or to sit without vibration on a tripod or other support.

Most cameras with a self-timer function flash a light or LED during the countdown, emit a beeping sound, or both. These warnings generally increase in speed or intensity as the count reaches the last few seconds, to alert that the shutter is about to fire.

The most common delay provided with a self-timer function is 10 seconds. Some cameras also give a 2 second setting, or an adjustable setting. Very few cameras can be set up to fire at a specified time.

Single-lens-reflex cameras have to flip up the viewing mirror before the picture is taken, which can also shake the camera. It is not uncommon for a camera to combine mirror lockup with the 2-second self-timer mode, which is generally used to reduce camera shake still further.

Alternatives to the use of the self-timer function include forms of remote shutter release, including a cable release, an infrared remote control, or other means.

Double Exposure

2008-06-13T02:04:00.003+08:00

Double exposure is a technique in which a piece of film is exposed twice, to two different images. The resulting photographic image shows the second image superimposed over the first.

The technique can be used to create ghostly images or to add people and objects to a scene that were not originally there. It is frequently used in photographic hoaxes.

It also is sometimes used as an artistic visual effect, especially when filming singers or musicians.

It is considered easiest to have a manual winding camera for double exposures. On automatic winding cameras, as soon as a picture is taken the film is typically wound to the next frame.

Some more advanced automatic winding cameras have the option for multiple exposures but it must be set before making each exposure.

Manual winding cameras with a multiple exposure feature can be set to double-expose after making the first exposure.

Since shooting multiple exposures will expose the same frame multiple times, negative exposure compensation must first be set to avoid overexposure.

For example, to expose the frame twice with correct exposure, a -1 EV compensation have to be done, and -2 EV for exposing four times.

This may not be necessary when photographing a lit subject in two (or more) different positions against a perfectly dark background, as the background area will be essentially unexposed.

Medium to low light is ideal for double exposures. A tripod may not be necessary if combining different scenes in one shot.

In some conditions, for example, recording the whole progress of a lunar eclipse in multiple exposures, a stable tripod is essential.

More than two exposures can be combined, with care not to overexpose the film.

Bracketing Exposure

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Without further qualifications, the term bracketing usually refers to exposure bracketing: the photographer chooses to take one picture at a given exposure, one or two brighter, and one or two darker, in order to select the most satisfactory image.

Many professional and advanced amateur cameras, including digital camera, can automatically shoot a bracketed series of pictures.

Exposure bracketing is indicated when dealing with high-contrast subjects and/or media with limited dynamic range, such as transparency film or CCD sensors in many digital cameras.

When shooting using print film, the person printing the pictures to paper must not compensate for the deliberately underexposed and overexposed pictures.

If a set of photos are bracketed but are then printed using automated equipment, the equipment may assume that the camera or photographer made an error and automatically "correct" the shots it determines are "improperly" done.

Highlights

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Areas of a photo where information is lost due to extreme brightness are described as having "blown-out highlights" or "flared highlights".

In digital images this information loss is often irreversible, though small problems can be made less noticeable using photo manipulation software. Recording to RAW format can ameliorate this problem to some degree, as can using a digital camera with a better sensor.

Film can often have areas of extreme overexposure but still record detail in those areas. This information is usually somewhat recoverable when printing or transferring to digital.

A loss of highlights in a photograph is usually undesirable, but in some cases can be considered to "enhance" appeal. Examples include black-and-white photography and portraits with an out-of-focus background.

Example image exhibiting blown-out highlights.
Top: original image,
Bottom: blown-out areas marked red

Black

Areas of a photo where information is lost due to extreme darkness are described as "crushed blacks". Digital capture tends to be more tolerant of underexposure, allowing better recovery of shadow detail, than same-ISO negative print film.

Crushed blacks cause loss of detail, but can be used for artistic effect.

Latitude

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Latitude is the degree by which you can over, or under expose an image, and still recover an acceptable level of quality from an exposure.

Typically negative film has a better ability to record a range of brightness than slide/transparency film or digital.

Digital should be considered to be the reverse of print film, with a good latitude in the shadow range, and a narrow one in the highlight area; in contrast to film's large highlight latitude, and narrow shadow latitude.

Slide/Transparency film has a narrow latitude in both highlight and shadow areas, requiring greater exposure accuracy.

Negative film's latitude increases somewhat with high ISO material, in contrast digital tends to narrow on latitude with high ISO settings.

A photograph of the Forth Rail Bridge
with an exposure time of 13 seconds
- the effect of a long exposure shot
on moving water is to make it
seem creamy and opalescent.

Determining Exposure

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The zone system is another method of determining exposure and development combinations to achieve a greater tonality range over conventional methods by varying the contrast of the 'film' to fit the print contrast capability.

Digital cameras can achieve similar results (high dynamic range) by combining several different exposures (varying only the shutter speeds) made in quick succession.

Today, most cameras automatically determine the correct exposure at the time of taking a photograph by using a built-in light meter, or multiple point meters interpreted by a built-in computer.

Negative/Print film tends to bias for exposing for the shadow areas (film dislikes being starved of light), with digital favouring exposure for highlights.

Reciprocity

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An important principle of exposure is reciprocity. If one exposes the film or sensor for a longer period, a reciprocally smaller aperture is required to reduce the amount of light hitting the film to obtain the same exposure. For example, the photographer may prefer to make his sunny-16 shot at an aperture of f/5.6 (to obtain a shallow depth of field). As f/5.6 is 3 stops 'faster' than f/16, with each stop meaning double the amount of light, a new shutter speed of (1/125)/(2·2·2) = 1/1000 is needed. Once the photographer has determined the exposure, aperture stops can be traded for halvings or doublings of speed, within limits.

A demonstration of the effect of exposure in night photography.
Longer shutter speeds mean increased exposure.

The true characteristic of most photographic emulsions is not actually linear, but it is close enough over the exposure range of about one second to 1/1000th of a second. Outside of this range, it becomes necessary to increase the exposure from the calculated value to account for this characteristic of the emulsion. This characteristic is known as reciprocity failure. The film manufacturer's data sheets should be consulted to arrive at the correction required as different emulsions have different characteristics.

Digital camera image sensors can also be subject to a form of reciprocity failure.

Correct Exposure

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The "correct" exposure for a photograph is determined by the sensitivity of the medium used. For photographic film, sensitivity is referred to as film speed and is measured on a scale published by the International Organization for Standardization (ISO).

Faster film requires less exposure and has a higher ISO rating. Exposure is a combination of the length of time and the level of illumination received by the photosensitive material. Exposure time is controlled in a camera by shutter speed and the illumination level by the lens aperture.

Slower shutter speeds (exposing the medium for a longer period of time) and greater lens apertures (admitting more light) produce greater exposures.

An approximately correct exposure will be obtained on a sunny day using ISO 100 film, an aperture of f/16 and a shutter speed of 1/100th of a second. This is called the sunny 16 rule: at an aperture of f/16 on a sunny day, a suitable shutter speed will be one over the film speed (or closest equivalent).

Ultimately there is no such thing as "correct exposure", as a scene can be exposed in many ways, depending on the desired effect a photographer wishes to convey.

A photograph with an exposure
time of 25 seconds

A photograph of a night-time sky with
an exposure time of 8 seconds

A two second exposure of
a fire poi ball dance

Exposure

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In photography, exposure is the total amount of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a photograph.

Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance.

Derivation of the DOF Formulas

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DOF for symmetrical lens.

DOF limits
A symmetrical lens is illustrated at right. The subject, at distance $s$ , is in focus at image distance $v$ . Point objects at distances $D F$ and $D N$ would be in focus at image distances $v F$ and $v N$ , respectively; at image distance $v$ , they are imaged as blur spots. The depth of field is controlled by the aperture stop diameter $d$ ; when the blur spot diameter is equal to the acceptable circle of confusion $c$ , the near and far limits of DOF are at $D N$ and $D F$ . From similar triangles,

It usually is more convenient to work with the lens f-number than the aperture diameter; the f-number $N$ is related to the lens focal length $f$ and the aperture diameter $d$ by

substituting into the previous equations and rearranging gives

The image distance $v$ is related to an object distance $s$ by the thin lens equation

Substituting into the two previous equations and rearranging gives the near and far limits of DOF:

Hyperfocal distance

Setting the far limit of DOF $D F$ to infinity and solving for the focus distance $s$ gives

where $H$ is the hyperfocal distance. Setting the subject distance to the hyperfocal distance and solving for the near limit of DOF gives

For any practical value of $H$ , the focal length is negligible in comparison, so that

Substituting the approximate expression for hyperfocal distance into the formulas for the near and far limits of DOF gives

Combining, the depth of field $D F - D N$ is

Moderate-to-large distances

When the subject distance is large in comparison with the lens focal length,

For , the far limit of DOF is at infinity and the DOF is infinite; of course, only objects at or beyond the near limit of DOF will be recorded with acceptable sharpness.

Close-up

When the subject distance $s$ approaches the lens focal length, the focal length no longer is negligible, and the approximate formulas above cannot be used without introducing significant error. At close distances, the hyperfocal distance has little applicability, and it usually is more convenient to express DOF in terms of magnification. Substituting

and

into the formula for DOF and rearranging gives

At the hyperfocal distance, the terms in the denominator are equal, and the DOF is infinite. As the subject distance decreases, so does the second term in the denominator; when , the second term becomes small in comparison with the first, and

so that for a given magnification, DOF is independent of focal length. Stated otherwise, for the same subject magnification, all focal lengths for a given image format give approximately the same DOF. This statement is true only when the subject distance is small in comparison with the hyperfocal distance, however. Multiplying the numerator and denominator of the exact formula by

gives

Decreasing the focal length $f$ increases the second term in the denominator, decreasing the denominator and increasing the value of the right-hand side, so that a shorter focal length gives greater DOF. The effect of focal length is greatest near the hyperfocal distance, and decreases as subject distance is decreased. However, the near/far perspective will differ for different focal lengths, so the difference in DOF may not be readily apparent. When the subject distance is small in comparison with the hyperfocal distance, the effect of focal length is negligible, and, as noted above, the DOF essentially is independent of focal length.

Near : Far DOF ratio

From the “exact” equations for near and far limits of DOF, the DOF in front of the subject is

and the DOF beyond the subject is

The near:far DOF ratio is

This ratio is always less than unity; at moderate-to-large subject distances, , and

When the subject is at the hyperfocal distance or beyond, the far DOF is infinite, and the near:far ratio is zero. It's commonly stated that approximately 1/3 of the DOF is in front of the subject and approximately 2/3 is beyond; however, this is true only when .

At closer subject distances, it's often more convenient to express the DOF ratio in terms of the magnification

Substitution into the “exact” equation for DOF ratio gives

As magnification increases, the near:far ratio approaches a limiting value of unity.

Focus and `f`-number from DOF limits

Not all images require that sharpness extend to infinity; the equations for the DOF limits can be combined to eliminate $N c$ and solve for the subject distance. For given near and far DOF limits $D N$ and $D F$ , the subject distance is

The equations for DOF limits also can be combined to eliminate $s$ and solve for the required f-number, giving

When the subject distance is large in comparison with the lens focal length, this simplifies to

Most discussions of DOF concentrate on the object side of the lens, but the formulas are simpler and the measurements usually easier to make on the image side. If $v N$ and $v F$ are the image distances that correspond to the near and far limits of DOF, the required f-number is minimum when the image distance $v$ is

The required f-number is

which require only the difference between the near and far image distances; focus is simply set to halfway between the near and far distances. View camera users often refer to the difference as the focus spread; it usually is measured on the bed or focusing rail. On manual-focus small- and medium-format lenses, the focus and f-number usually are determined using the lens DOF scales, which often are based on the two equations above.

For close-up photography, the f-number is more accurately determined using

where $m$ is the magnification.

Defocus blur for background object at B.

Foreground and background blur

If the equation for the far limit of DOF is solved for $c$ , and the far distance replaced by an arbitrary distance $D$ , the blur disk diameter $b$ at that distance is

When the background is at the far limit of DOF, the blur disk diameter is equal to the circle of confusion $c$ , and the blur is just imperceptible. The diameter of the background blur disk increases with the distance to the background. A similar relationship holds for the foreground; the general expression for a defocused object at distance $D$ is

For a given scene, the distance between the subject and a foreground or background object is usually fixed; let that distance be represented by

then

or, in terms of subject distance,

with the minus sign used for foreground objects and the plus sign used for background objects. For a relatively distant background object,

In terms of subject magnification, the subject distance is

so that, for a given f-number and subject magnification,

Differentiating $b$ with respect to $f$ gives

With the plus sign, the derivative is everywhere positive, so that for a background object, the blur disk size increases with focal length. With the minus sign, the derivative is everywhere negative, so that for a foreground object, the blur disk size decreases with focal length.

The magnification of the defocused object also varies with focal length; the magnification of the defocused object is

where $v s$ is the image distance of the subject. For a defocused object with some characteristic dimension $y$ , the imaged size of that object is

The ratio of the blur disk size to the imaged size of that object then is

so for a given defocused object, the ratio of the blur disk diameter to object size is independent of focal length, and depends only on the object size and its distance from the subject.

Asymmetrical lenses

The discussion thus far has assumed a symmetrical lens for which the entrance and exit pupils coincide with the object and image nodal planes, and for which the pupil magnification is unity. Although this assumption usually is reasonable for large-format lenses, it often is invalid for medium- and small-format lenses.

For an asymmetrical lens, the DOF ahead of the subject distance and the DOF beyond the subject distance are given by

where $P$ is the pupil magnification.

Combining gives the total DOF:

When , the second term in the denominator becomes small in comparison with the first, and

When the pupil magnification is unity, the equations for asymmetrical lenses reduce to those given earlier for symmetrical lenses.

Effect of lens asymmetry

Except for close-up and macro photography, the effect of lens asymmetry is minimal. A slight rearrangement of the last equation gives

As magnification decreases, the

1 / P

term becomes smaller in comparison with the

1 / m

term, and eventually the effect of pupil magnification becomes negligible.

Digital Techniques For Increasing DOF

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At f/11, the DOF in this image of a Wolf
spider is very limited.

Combining 8 exposures, each taken at f/11,
gives good DOF.

Focus Stacking
Focus stacking is a digital image processing technique which combines multiple images taken at different focus distances to give a resulting image with a greater depth of field than any of the individual source images. Available programs for multi-shot DOF enhancement include Syncroscopy AutoMontage, PhotoAcute Studio, Helicon Focus and CombineZM.

Wavefront Coding
Wavefront coding is a method that convolves rays in such a way to provide an image where fields are in focus simultaneously with all planes out of focus by a constant amount.

Plenoptic Cameras
A plenoptic camera uses a microlens array to capture 4D light field information about a scene.

DOF vs Format Size

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To a first approximation, DOF is inversely proportional to format size. More precisely, if photographs with the same final-image size are taken in two different camera formats at the same subject distance with the same field of view and f-number, the DOF is, to a first approximation, inversely proportional to the format size. Strictly speaking, this is true only when the subject distance is large in comparison with the focal length and small in comparison with the hyperfocal distance, for both formats, but it nonetheless is generally useful for comparing results obtained from different formats.

To maintain the same field of view, the lens focal lengths must be in proportion to the format sizes. Assuming, for purposes of comparison, that the 4×5 format is four times the size of 35 mm format, if a 4×5 camera used a 300 mm lens, a 35 mm camera would need a 75 mm lens for the same field of view. For the same f-number, the image made with the 35 mm camera would have four times the DOF of the image made with the 4×5 camera.

In many cases, the DOF is fixed by the requirements of the desired image. For a given DOF and field of view, the required f-number is proportional to the format size. For example, if a 35 mm camera required f/11, a 4×5 camera would require f/45 to give the same DOF. For the same ISO speed, the exposure time on the 4×5 would be sixteen times as long; if the 35 camera required 1/250 second, the 4×5 camera would require 1/15 second. In windy conditions, the exposure time with the larger camera might allow motion blur. Adjusting the f-number to the camera format is equivalent to maintaining the same absolute aperture diameter.

The greater DOF with the smaller format can be either an advantage or a disadvantage, depending on the desired effect. For the same amount of foreground and background blur, a small-format camera requires a smaller f-number and allows a shorter exposure time than a large-format camera; however, many point-and-shoot digital cameras cannot provide a very shallow DOF. For example, a point-and-shoot digital camera with a 1/1.8″ sensor (7.18 mm × 5.32 mm) at a normal focal length and f/2.8 has the same DOF as a 35 mm camera with a normal lens at f/13.

In some cases, camera movements (tilt or swing) can be used to better fit the DOF to the scene, and achieve the required sharpness at a smaller f-number.

Depth of Field Formulas

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The basis of these formulas is given in the section Derivation of the DOF formulas refer to the diagram in that section for illustration of the quantities discussed below.

Hyperfocal Distance
Let $f$ be the lens focal length, $N$ be the lens f-number, and $c$ be the circle of confusion for a given image format. The hyperfocal distance $H$ is given by

Moderate-to-large distances
Let $s$ be the distance at which the camera is focused (the “subject distance”). When $s$ is large in comparison with the lens focal length, the distance $D N$ from the camera to the near limit of DOF and the distance $D F$ from the camera to the far limit of DOF are

When the subject distance is the hyperfocal distance,

The depth of field $D F - D N$ is

For , the far limit of DOF is at infinity and the DOF is infinite; of course, only objects at or beyond the near limit of DOF will be recorded with acceptable sharpness.

Substituting for $H$ and rearranging, DOF can be expressed as

Thus, for a given image format, depth of field is determined by three factors: the focal length of the lens, the f-number of the lens opening (the aperture), and the camera-to-subject distance.

Close-up
When the subject distance $s$ approaches the focal length, using the formulas given above can result in significant errors. For close-up work, the hyperfocal distance has little applicability, and it usually is more convenient to express DOF in terms of image magnification. Let $m$ be the magnification; when the subject distance is small in comparison with the hyperfocal distance,

so that for a given magnification, DOF is independent of focal length. Stated otherwise, for the same subject magnification, all focal lengths give approximately the same DOF. This statement is true only when the subject distance is small in comparison with the hyperfocal distance, however.

The discussion thus far has assumed a symmetrical lens for which the entrance and exit coincide with the front and rear pupils nodal planes, and for which the pupil magnification (the ratio of exit pupil diameter to that of the entrance pupil is unity. Although this assumption usually is reasonable for large-format lenses, it often is invalid for medium- and small-format lenses.

When , the DOF for an asymmetrical lens is

where $P$ is the pupil magnification. When the pupil magnification is unity, this equation reduces to that for a symmetrical lens.

Except for close-up and macro photography, the effect of lens asymmetry is minimal. At unity magnification, however, the errors from neglecting the pupil magnification can be significant. Consider a telephoto lens with $P = 0.5$ and a retrofocus wide-angle lens with $P = 2$ , at $m = 1.0$ . The asymmetrical-lens formula gives $DOF = 6 N c$ and $DOF = 3 N c$ , respectively. The symmetrical-lens formula gives $DOF = 4 N c$ in either case. The errors are −33% and 33%, respectively.

Focus and f-number from DOF limits
Not all images require that sharpness extend to infinity; for given near and far DOF limits $D N$ and $D F$ , the required f-number is smallest when focus is set to

When the subject distance is large in comparison with the lens focal length, the required f-number is

In practice, these settings usually are determined on the image side of the lens, using measurements on the bed or rail with a view camera, or using lens DOF scales on manual-focus lenses for small- and medium-format cameras. If $v N$ and $v F$ are the image distances that correspond to the near and far limits of DOF, the required f-number is minimized when the image distance $v$ is

In practical terms, focus is set to halfway between the near and far image distances. The required f-number is

The image distances are measured from the camera's image plane to the lens's image nodal plane, which is not always easy to locate. In most cases, focus and f-number can be determined with sufficient accuracy using the approximate formulas above, which require only the difference between the near and far image distances; view camera users often refer to the difference as the focus spread. Most lens DOF scales are based on the same concept.

Foreground and background blur
If a subject is at distance $s$ and the foreground or background is at distance $D$ , let the distance between the subject and the foreground or background be indicated by

The blur disk diameter $b$ of a detail at distance $x d$ from the subject can be expressed as a function of the focal length $f$ , subject magnification $m s$ , and f-number $N$ according to

The minus sign applies to a foreground object, and the plus sign applies to a background object.

The blur increases with the distance from the subject; when , the detail is within the depth of field, and the blur is imperceptible. If the detail is only slightly outside the DOF, the blur may be only barely perceptible.

For a given subject magnification, f-number, and distance from the subject of the foreground or background detail, the degree of detail blur varies with the lens focal length. For a background detail, the blur increases with focal length; for a foreground detail, the blur decreases with focal length. For a given scene, the positions of the subject, foreground, and background usually are fixed, and the distance between subject and the foreground or background remains constant regardless of the camera position; however, to maintain constant magnification, the subject distance must vary if the focal length is changed. For small distance between the foreground or background detail, the effect of focal length is small; for large distance, the effect can be significant. For a reasonably distant background detail, the blur disk diameter is

depending only on focal length.

The blur diameter of foreground details is very large if the details are close to the lens.

The ratio $b / c$ is independent of camera format; the blur then is in terms of circles of confusion.

The magnification of the detail also varies with focal length; for a given detail, the ratio of the blur disk diameter to imaged size of the detail is independent of focal length, depending only on the detail size and its distance from the subject. This ratio can be useful when it is important that the background be recognizable (as usually is the case in evidence or surveillance photography), or unrecognizable (as might be the case for a pictorial photographer using selective focus to isolate the subject from a distracting background). As a general rule, an object is recognizable if the blur disk diameter is one-tenth to one-fifth the size of the object or smaller and unrecognizable when the blur disk diameter is the object size or greater.

The effect of focal length on background blur is illustrated in van Walree's article on Depth of Field.

Practical complications
The distance scales on most medium- and small-format lenses indicate distance from the camera's image plane. Most DOF formulas, including those in this article, use the object distance $s$ from the lens's object nodal plane, which often is not easy to locate. Moreover, for many zoom lenses and internal-focusing non-zoom lenses, the location of the object nodal plane, as well as focal length, changes with subject distance. When the subject distance is large in comparison with the lens focal length, the exact location of the object nodal plane is not critical; the distance is essentially the same whether measured from the front of the lens, the image plane, or the actual nodal plane. The same is not true for close-up photography; at unity magnification, a slight error in the location of the object nodal plane can result in a DOF error greater than the errors from any approximations in the DOF equations.

The asymmetrical lens formulas require knowledge of the pupil magnification, which usually is not specified for medium- and small-format lenses. The pupil magnification can be estimated by looking into the front and rear of the lens and measuring the diameters of the apparent apertures, and computing the ratio (rear diameter divided by front diameter). However, for many zoom lenses and internal-focusing non-zoom lenses, the pupil magnification changes with subject distance, and several measurements may be required.

Limitations
Most DOF formulas, including those discussed in this article, employ several simplifications:

Paraxial (Gaussian) optics is assumed, and technically, the formulas are valid only for rays that are infinitesimally close to the lens axis. However, Gaussian optics usually is more than adequate for determining DOF, and non-paraxial formulas are sufficiently complex that requiring their use would make determination of DOF impractical in most cases.
Lens aberrations are ignored. Including the effects of aberrations is nearly impossible, because doing so requires knowledge of the specific lens design. Moreover, in well-designed lenses, most aberrations are well corrected, and at least near the optical axis, often are almost negligible when the lens is stopped down 2–3 steps from maximum aperture. Because lenses usually are stopped down at least to this point when DOF is of interest, ignoring aberrations usually is reasonable. Not all aberrations are reduced by stopping down, however, so actual sharpness may be slightly less than predicted by DOF formulas.
Diffraction is ignored. DOF formulas imply that any arbitrary DOF can be achieved by using a sufficiently large f-number . Because of diffraction, however, this isn't quite true. Once a lens is stopped down to where most aberrations are well corrected, stopping down further will decrease sharpness in the center of the field. At the DOF limits, however, further stopping down decreases the size of the defocus blur spot, and the overall sharpness may increase. Consequently, choosing an f-number sometimes involves a tradeoff between center and edge sharpness, although viewers typically prefer uniform sharpness to slightly greater center sharpness. The choice, of course, is subjective, and may depend upon the particular image. Eventually, the defocus blur spot becomes negligibly small, and further stopping down serves only to decrease sharpness even at DOF limits. Typically, diffraction at DOF limits becomes significant only at fairly large f-numbers; because large f-numbers typically require long exposure times, motion blur often causes greater loss of sharpness than does diffraction.
Post-capture manipulation of the image is ignored. Sharpening via techniques such as deconvolution or unsharp mask can increase the DOF in the final image, particularly when the original image has a large DOF. Conversely, image noise reduction can reduce the DOF.
For digital capture with color filter array sensors, demosaicing is ignored. Demosaicing alone would normally reduce the DOF, but the demosaicing algorithm used might also include sharpening.

The lens designer cannot restrict analysis to Gaussian optics and cannot ignore lens aberrations. However, the requirements of practical photography are less demanding than those of lens design, and despite the simplifications employed in development of most DOF formulas, these formulas have proven useful in determining camera settings that result in acceptably sharp pictures. It should be recognized that DOF limits are not hard boundaries between sharp and unsharp, and that there is little point in determining DOF limits to a precision of many significant figures.

Near : Far Distribution

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The DOF beyond the subject is always greater than the DOF in front of the subject.

When the subject is at the hyperfocal distance or beyond, the far DOF is infinite; as the subject distance decreases, near:far DOF ratio increases, approaching unity at high magnification.

The oft-cited “rule” that 1/3 of the DOF is in front of the subject and 2/3 is beyond is true only when the subject distance is 1/3 the hyperfocal distance.

Limited DOF: Selective Focus

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Depth of field can be anywhere from a fraction of a millimeter to virtually infinite. In some cases, such as landscapes, it may be desirable to have the entire image in focus, and a large DOF is appropriate.

In other cases, artistic considerations may dictate that only a part of the image be in focus, emphasizing the subject while de-emphasizing the background, perhaps giving only a suggestion of the environment (Langford 1973, 81).

For example, a common technique in melodramas and horror films is a closeup of a person's face, with someone just behind that person visible but out of focus. A portrait or closeup still photograph might use a small DOF to isolate the subject from a distracting background. The use of limited DOF to emphasize one part of an image is known as selective focus or differential focus.

Although a small DOF implies that other parts of the image will be unsharp, it does not, by itself, determine how unsharp those parts will be. The amount of background (or foreground) blur depends on the distance from the plane of focus, so if a background is close to the subject, it may be difficult to blur sufficiently even with a small DOF. In practice, the lens f-number is usually adjusted until the background or foreground is acceptably blurred, often without direct concern for the DOF.

Sometimes, however, it is desirable to have the entire subject sharp while ensuring that the background is sufficiently unsharp. When the distance between subject and background is fixed, as is the case with many scenes, the DOF and the amount of background blur are not independent. Although it is not always possible to achieve both the desired subject sharpness and the desired background unsharpness, several techniques can be used to increase the separation of subject and background.

For a given scene and subject magnification, the background blur increases with lens focal length. If it is not important that background objects be unrecognizable, background de-emphasis can be increased by using a lens of longer focal length and increasing the subject distance to maintain the same magnification. This technique requires that sufficient space in front of the subject be available; moreover, the perspective of the scene changes because of the different camera position, and this may or may not be acceptable.

The situation is not as simple if it is important that a background object, such as a sign, be unrecognizable. The magnification of background objects also increases with focal length, so with the technique just described, there is little change in the recognizability of background objects. However, a lens of longer focal length may still be of some help; because of the narrower angle of view, a slight change of camera position may suffice to eliminate the distracting object from the field of view.

Although tilt and swing are normally used to maximize the part of the image that is within the DOF, they also can be used, in combination with a small f-number, to give selective focus to a plane that isn't perpendicular to the lens axis. With this technique, it is possible to have objects at greatly different distances from the camera in sharp focus and yet have a very shallow DOF. The effect can be interesting because it differs from what most viewers are accustomed to seeing.

Obtaining Maximum DOF

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Lens DOF Scales

Many lenses for small- and medium-format cameras include scales that indicate the DOF for a given focus distance and f-number; the 35 mm Nikkor lens in the image above is typical. That lens includes distance scales in feet and meters; when a marked distance is set opposite the large white index mark, the focus is set to that distance.

The DOF scale below the distance scales includes markings on either side of the index that correspond to f-numbers; when the lens is set to a given f-number, the DOF extends between the distances that align with the f-number markings.

Zone Focusing
When the 35 mm lens above is set to f / 11 and focused at approximately 1.4 m, the DOF (a “zone” of acceptable sharpness) extends from 1 m to 2 m. Conversely, the required focus and f-number can be determined from the desired DOF limits by locating the near and far DOF limits on the lens distance scale and setting focus so that the index mark is centered between the near and far distances; the required f-number is determined by finding the markings on the DOF scale that are closest to the near and far distances.

For the 35 mm lens above, if it were desired for the DOF to extend from 1 m to 2 m, focus would be set to approximately 1.4 m and the aperture set to f / 11. The DOF limits can be determined from a scene by focusing on the farthest object to be within the DOF and noting the distance on the lens distance scale, and repeating the process for the nearest object to be within the DOF. If the near and far distances fall outside the largest f-number markings on the DOF scale, the desired DOF cannot be obtained; for example, with the 35 mm lens above, it is not possible to have the DOF extend from 0.7 m to infinity.

Some distance scales have markings for only a few distances; for example, the 35 mm lens above shows only 3 ft and 5 ft on its upper scale. Using other distances for DOF limits requires visual interpolation between marked distances; because the distance scale is nonlinear, accurate interpolation can be difficult. In most cases, English and metric distance markings are not coincident, so using both scales to note focused distances can sometimes lessen the need for interpolation.

Many autofocus lenses have smaller distance and DOF scales and fewer markings than do comparable manual-focus lenses, so that determining focus and f-number from the scales on an autofocus lens may be more difficult than with a comparable manual-focus lens. In most cases, using the lens DOF scales on an autofocus lens requires that the lens or camera body be set to manual focus.

On a view camera, the focus and f-number can be obtained by measuring the focus spread and performing simple calculations; the procedure is described in more detail in the section Focus and f-number from DOF limits. Some view cameras include DOF calculators that indicate focus and f-number without the need for any calculations by the photographer.

Hyperfocal Distance
The hyperfocal distance is the nearest focus distance at which the DOF extends to infinity; focusing the camera at the hyperfocal distance results in the largest possible depth of field for a given f-number.

Focusing beyond the hyperfocal distance does not increase the far DOF (which already extends to infinity), but it does decrease the DOF in front of the subject, decreasing the total DOF. Some photographers refer to this as “wasting DOF”; however, see The object field method below for a rationale for doing so. If the lens includes a DOF scale, the hyperfocal distance can be set by aligning the infinity mark on the distance scale with the mark on the DOF scale corresponding to the f-number to which the lens is set.

For example, with the 35 mm lens shown above set to f / 11, aligning the infinity mark with the ‘11’ to the left of the index mark on the DOF scale would set the focus to the hyperfocal distance. Focusing on the hyperfocal distance is a special case of zone focusing in which the far limit of DOF is at infinity.

The Object Field Method
Traditional depth-of-field formulas and tables assume equal circles of confusion for near and far objects. Some authors, such as Merklinger (1992), have suggested that distant objects often need to be much sharper to be clearly recognizable, whereas closer objects, being larger on the film, do not need to be so sharp.

The loss of detail in distant objects may be particularly noticeable with extreme enlargements. Achieving this additional sharpness in distant objects usually requires focusing beyond the hyperfocal distance, sometimes almost at infinity. For example, if photographing a cityscape with a traffic bollard in the foreground, this approach, termed the object field method by Merklinger, would recommend focusing very close to infinity, and stopping down to make the bollard sharp enough.

With this approach, foreground objects cannot always be made perfectly sharp, but the loss of sharpness in near objects may be acceptable if recognizability of distant objects is paramount.

Moritz Von Rohr also used an object field method, but unlike Merklinger, he used the conventional criterion of a maximum circle of confusion diameter in the image plane, leading to unequal front and rear depths of field.

At f/32, the background
is distracting.

At f/5.6, the flowers are isolated
from the background.

Camera Movements and DOF

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When the lens axis is perpendicular to the image plane, as is normally the case, the Plane of Focus (POF) is parallel to the image plane, and the DOF extends between parallel planes on either side of the POF.

When the lens axis is not perpendicular to the image plane, the POF is no longer parallel to the image plane; the ability to rotate the POF is known as the Scheimpflug principle.

Rotation of the POF is accomplished with camera movements (tilt, a rotation of the lens about a horizontal axis, or swing, a rotation about a vertical axis).

Tilt and swing are available on most view cameras, and are also available with specific lenses on some small- and medium-format cameras.

When the POF is rotated, the near and far limits of DOF are no longer parallel; the DOF becomes wedge-shaped, with the apex of the wedge nearest the camera.

With tilt, the height of the DOF increases with distance from the camera; with swing, the width of the DOF increases with distance.

Rotating the POF with tilt or swing (or both) can be used either to maximize or minimize the part of an image that is within the DOF.

Effect of Lens Aperture

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For a given subject framing and camera position, the DOF is controlled by the lens aperture diameter, which is usually specified as the f-number, the ratio of lens focal length to aperture diameter. Reducing the aperture diameter (increasing the f-number) increases the DOF; however, it also reduces the amount of light transmitted, and increases diffraction, placing a practical limit on the extent to which DOF can be increased by reducing the aperture diameter.

Motion pictures make only limited use of this control; to produce a consistent image quality from shot to shot, cinematographers usually choose a single aperture setting for interiors and another for exteriors, and adjust exposure through the use of camera filters or light levels. Aperture settings are adjusted more frequently in still photography, where variations in depth of field are used to produce a variety of special effects.

f/22

f/8

f/4

f/2.8

Apparent Sharpness

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Precise focus is possible at only one distance; at that distance, a point object will produce a point image. At any other distance, a point object is defocused, and will produce a blur spot shaped like the aperture, which for the purpose of analysis is usually assumed to be circular.

When this circular spot is sufficiently small, it is indistinguishable from a point, and appears to be in focus; it is rendered as “acceptably sharp”.

The diameter of the circle increases with distance from the point of focus; the largest circle that is indistinguishable from a point is known as the acceptable circle of confusion , or informally, simply as the circle of confusion.

The acceptable circle of confusion is influenced by visual acuity, viewing conditions, and the amount by which the image is enlarged. The increase of the circle diameter with defocus is gradual, so the limits of depth of field are not hard boundaries between sharp and unsharp.

Several other factors, such as subject matter, movement, and the distance of the subject from the camera, also influence when a given defocus becomes noticeable.

For a 35mm motion picture, the image area on the negative is roughly 22 mm by 16 mm (0.87 in by 0.63 in). The limit of tolerable error is usually set at 0.05 mm (0.002 in) diameter.

For 16mm film, where the image area is smaller, the tolerance is stricter, 0.025 mm (0.001 in). Standard depth-of-field tables are constructed on this basis, although generally 35 mm productions set it at 0.025 mm (0.001 in).

Note that the acceptable circle of confusion values for these formats are different because of the relative amount of magnification each format will need in order to be projected on a full-sized movie screen. (A table for 35 mm still photography would be somewhat different since more of the film is used for each image and the amount of enlargement is usually much less).

The image format size also will affect the depth of field. The larger the format size, the longer a lens will need to be to capture the same framing as a smaller format.

In motion pictures, for example, a frame with a 12 degree horizontal field of view will require a 50 mm lens on 16 mm film, a 100 mm lens on 35 mm film, and a 250 mm lens on 65 mm film.

Conversely, using the same focal length lens with each of these formats will yield a progressively wider image as the film format gets larger: a 50 mm lens has a horizontal field of view of 12 degrees on 16 mm film, 23.6 degrees on 35 mm film, and 55.6 degrees on 65 mm film.

What this all means is that because the larger formats require longer lenses than the smaller ones, they will accordingly have a smaller depth of field. Therefore, compensations in exposure, framing, or subject distance need to be made in order to make one format look like it was filmed in another format.

The area within the depth of field appears sharp while the areas in front of
and beyond the depth of field appear blurry.

Depth of Field (DOF)

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In optics, particularly as it relates to film and photography, the depth of field (DOF) is the portion of a scene that appears sharp in the image.

Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on either side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.

For some images, such as landscapes, a large DOF may be appropriate, while for others, such as portraits, a small DOF may be more effective.

The DOF is determined by the subject distance (that is, the distance to the plane that is perfectly in focus), the lens focal length, and the lens f-number (relative aperture). Except at close-up distances, DOF is approximately determined by the subject magnification and the lens f-number.

For a given f-number, increasing the magnification, either by moving closer to the subject or using a lens of greater focal length, decreases the DOF; decreasing magnification increases DOF. For a given subject magnification, increasing the f-number (decreasing the aperture diameter) increases the DOF; decreasing f-number decreases DOF.

When focus is set to the hyperfocal distance, the DOF extends from half the hyperfocal distance to infinity, and is the largest DOF possible for a given f-number.

The advent of digital technology in photography has provided additional means of controlling the extent of image sharpness; some methods allow DOF that would be impossible with traditional techniques, and some allow the DOF to be determined after the image is made.

Effect of aperture on blur and DOF. The points in focus (2) project points onto the image plane (5), but points at different distances (1 and 3) project blurred images, or circles of confusion. Decreasing the aperture size (4) reduces the size of the blur circles for points not in the focused plane, so that the blurring is imperceptible, and all points are within the DOF.

Flash Technique

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A flash is commonly used indoors as the main light source when there is not enough ambient light for a desired shutter speed.

A fill flash or fill-in flash is a low powered flash added to ambient light to illuminate a subject close to the camera while using an exposure long enough to capture background detail.

Another technique is to point a flash upwards onto a reflective surface, which may be a white ceiling or a flash umbrella, which reflects light onto the subject; this is called bounce flash.

Bouncing creates a more natural light effect than direct flash without glare in the highlights and impenetrable shadows, but requires more flash power than a direct flash.

Part of the bounced light can be also aimed directly on the subject by "bounce cards" attached to the flash unit which increase the efficiency of the flash and illuminate shadows cast by light coming from the ceiling. It's also possible to use one's own palm for that purpose, resulting in warmer tones on the picture, as well as eliminating the need to carry additional accessories.

Some camera manufacturers may be considering the inclusion of a built-in bounce flash within the body of a camera with automated features to assist the user in obtaining a bounced light effect without spending time to set up and direct external flash devices.

Types of Flashes

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Flash bulbs

The earliest flashes consisted of a quantity of magnesium flash powder that was ignited by hand. Later, magnesium filaments were contained in flash bulbs, and electrically ignited by a contact in the camera shutter; such a bulb could only be used once, and was too hot to handle immediately after use, but the confinement of what would otherwise have amounted to a small explosion was an important advance.

A later innovation was coating flashbulbs with a plastic coating to improve spectral quality as well as providing protection from the rare occasion when a flashbulb would crack during a flash.

Flashbulbs took longer to achieve full brightness and burned for a longer duration than electronic flashes, and slower shutter speeds (typically from 1/10 to 1/50 of a second) were used on cameras to ensure proper synchronization. One of the most widely used flash bulbs up through the 1960s was the number 25.

This is the large (approximately 1 inch (25 mm) in diameter) flash bulb often shown used by newspapermen in period movies, usually attached to a press camera or a twin-lens reflex camera.

Flashcubes, Magicubes and Flipflash

In the late 1960s, Kodak improved their Instamatic camera line by replacing the individual flashbulb technology (used on early Instamatics) with the Flashcube.

Flashcubes consisted of four electrically fired flashbulbs with an integral reflector in a cube-shaped arrangement that allowed taking four images in a row. The flashcube automatically rotated 90 degrees to a fresh bulb upon advancing the film to the next exposure.

The later Magicube retained the four-bulb format, and was superficially similar to the original Flashcube. However, in the Magicube each bulb was set off by a plastic pin in the cube mount that released a cocked spring wire within the cube. This wire, in turn, struck a primer tube, at the base of the bulb, which contained a fulminating material. The fulminate ignited shredded zirconium foil in the flash and, thus, a battery was not required. Magicubes could also be set off by inserting a thin object, such as a key or paper clip, into one of the slots in the bottom of the cube.

Another common flashbulb-based device was the Flipflash which included ten or so bulbs in a single unit. The name derived from the fact that once half the flashes had been used up, the unit had to be flipped and re-inserted to use the remainder.

Modern flash technology

Today's flash units are often electronic xenon flash lamps. An electronic flash contains a tube filled with xenon gas, where electricity of high voltage is discharged to generate an electrical arc that emits a short flash of light. (A typical duration of the light impulse is 1/1000 second.)

As of 2003, the majority of cameras targeted for consumer use have an electronic flash unit built in. Another type of flash unit are microflashes, which are special, high-voltage flash units designed to discharge a flash of light with an exceptionally quick, sub-microsecond duration.

These are commonly used by scientists or engineers for examining extremely fast moving objects or reactions, famous for producing images of bullets tearing through objects like lightbulbs or balloons.

Studio flashes usually contain a modeling light, which is an incandescent light bulb placed close to the flash tube. The continuous illumination of a modeling light helps in visualizing the effect of the flash.

The strength of a flash device is often indicated in terms of a guide number , despite the fact that the published guide numbers of different units can not necessarily be directly compared.

Although they are not yet at the power levels to replace Xenon flash devices in still cameras, LEDs (specifically, high current flash LEDs from manufacturers like Lumileds or Seoul Semiconductor) have recently been used as flash sources in camera phones.

LEDs are expected to approach the power levels of Xenon in the near future and may replace built-in Xenon flashes in still cameras. The major advantages of LEDs over Xenon include low voltage operation, higher efficiency and extreme miniaturization.

Flash

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A flash is a device used in photography that produces an instantaneous flash of artificial light (typically around 1/1000 to 1/200 of a second) at a color temperature of about 5500 K to help illuminate a scene.

While flashes can be used for a variety of reasons (e.g., capturing quickly moving objects, creating a different temperature light than the ambient light) they are mostly used to illuminate scenes that do not have enough available light to adequately expose the photograph.

The term flash can either refer to the flash of light itself, or as a colloquialism for the electronic flash unit which discharges the flash of light. The vast majority of flash units today are electronic, having evolved from single-use flash-bulbs and flammable powders.

In lower-end consumer photography, flash units are commonly built directly into the camera, while higher-end cameras allow separate flash units to be mounted via a standardized accessory mount bracket often called a "hot shoe".

In professional studio photography, flashes often take the form of large, standalone units, or studio strobes, that are powered by either special battery packs or connected directly to the mains and synchronized with the camera from either a flash synchronization cable, radio transmitter, or are light-triggered, meaning that only one flash unit needs to be synchronized with the camera, which in turn triggers the other units.

Running water "frozen" by flash.

Creative Utility in Photography

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Shutter speed is one of several methods used to control the amount of light recorded by the camera's digital sensor or film. It is also used to manipulate the visual effects of the final image beyond its luminosity.

Slower shutter speeds are often selected to suggest movement in a still photograph of a moving subject.

Excessively fast shutter speeds can cause a moving subject to appear unnaturally frozen. For instance, a running person may be caught with both feet in the air with all indication of movement lost in the frozen moment.

When a slower shutter speed is selected, a longer time passes from the moment the shutter opens till the moment it closes. More time is available for movement in the subject to be recorded by the camera.

A slightly slower shutter speed will allow the photographer to introduce an element of blur, either in the subject, where, in our example, the feet, which are the fastest moving element in the frame, might be blurred while the rest remains sharp; or if the camera is panned to follow a moving subject, the background is blurred while the subject remains sharp.

The exact point at which the background or subject will start to blur depends on the rate at which the object is moving, the distance it is from the camera and the focal length of the lens in relation to the size of the digital sensor or film.

When slower shutter speeds, in excess of about half a second, are used on running water, the photo will have a ghostly white appearance reminiscent of fog. This effect can be used in landscape photography.

Zoom burst is a technique which entails the variation of the focal length of a zoom lens during a longer exposure. In the moment that the shutter is opened, the lens is zoomed in, changing the focal length during the exposure. The center of the image remains sharp, while the details away from the center form a radial blur, which causes a strong visual effect, forcing the eye into the center of the image.

Shutter speed can have a dramatic impact on the appearance of moving objects. Changes in background blurring are apparent from the need to adjust the aperture size to achieve proper exposure.

Slow shutter speed combined with panning the camera can achieve a motion blur for moving objects.

A photo of sparks coming from coals (exposure time 15 seconds)

Shutter Speed

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Shutter Speed is the length of time a shutter is open; the total exposure is proportional to this exposure time, or duration of light reaching the film or image sensor.

Factors that affect the total exposure of a photograph include the scene luminance, the aperture size (f-number), and the exposure time (shutter speed); photographers can trade off shutter speed and aperture by using units of stops.

A stop up and down on each will halve or double the amount of light regulated by each; exposures of equal exposure value can be easily calculated and selected.

For any given total exposure, or exposure value, a fast shutter speed requires a larger aperture (smaller f-number). Similarly, a slow shutter speed, a longer length of time, can be compensated by a smaller aperture (larger f-number).

Slow shutter speeds are often used in low light conditions, extending the time until the shutter closes, and increasing the amount of light gathered. This basic principle of photography, the exposure, is used in film and digital cameras, the image sensor effectively acting like film when exposed by the shutter.

Shutter speed is measured in seconds. A typical shutter speed for photographs taken in sunlight is 1/125th of a second.

In addition to its effect on exposure, shutter speed changes the way movement appears in the picture. Very short shutter speeds are used to freeze fast-moving subjects, for example at sporting events. Very long shutter speeds are used to intentionally blur a moving subject for artistic effect.

Adjustment to the aperture controls the depth of field, the distance range over which objects are acceptably sharp; such adjustments generally need to be compensated by changes in the shutter speed.In early days of photography, available shutter speeds were somewhat ad hoc.

Following the adoption of a standardized way of representing aperture so that each major step exactly doubled or halved the amount of light entering the camera (f/2.8, f/4, f/5.6, f/8, f/11, f/16, etc.), a standardized 2:1 scale was adopted for shutter speed so that opening one aperture stop and reducing the shutter speed by one step resulted in the identical exposure. The agreed standards for shutter speeds are:

1/1000 s
1/500 s
1/250 s
1/125 s
1/60 s
1/30 s
1/15 s
1/8 s
1/4 s
1/2 s
1 s

Each standard increment either doubles the amount of light (longer time) or halves the amount of light (shorter time). For example, if you move from 1 sec to 1/2 second, you have effectively halved the amount of light entering the shutter.

This scale can be extended at either end in specialist cameras. Some older cameras use the 2:1 ratio at slightly different values, such as 1/100 s and 1/50 s, although mechanical shutter mechanisms were rarely precise enough for the difference to have any significance.

The term "speed" is used in reference to short exposure times as fast, and long exposure times as slow. Shutter speeds are often designated by the reciprocal time, for example 60 for 1/60 s.

Camera shutters often include one or two other settings for making very long exposures:

B (for bulb) — keep the shutter open as long as the shutter release is held
T (for time) — keep the shutter open until the shutter release is pressed again

The ability of the photographer to take images without noticeable blurring by camera movement is an important parameter in the choice of slowest possible shutter speed for a handheld camera.

The rough guide used by most 35mm photographers is that the slowest shutter speed that can be used easily without much blur due to camera shake is the shutter speed numerically closest to the lens focal length. For example, for handheld use of a 35 mm camera with a 50 mm normal lens, the closest shutter speed is 1/60 s.

This rule can be augmented with knowledge of the intended application for the photograph, an image intended for significant enlargement and closeup viewing would require faster shutter speeds to avoid obvious blur.

Through practice and special techniques such as bracing the camera, arms, or body to minimize camera movement longer shutter speeds can be used without blur. If a shutter speed is too slow for hand holding, a camera support — usually a tripod — must be used. Image stabilization can often permit the use of shutter speeds 3-4 stops slower (exposures 8-16 times longer).

Shutter priority refers to a shooting mode used in semi-automatic cameras. It allows the photographer to choose a shutter speed setting and allow the camera to decide the correct aperture. This is sometimes referred to as Shutter Speed Priority Auto Exposure, or Tv (time value) mode.

Aperture Area

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The amount of light captured by a lens is proportional to the area of the aperture, equal to:

Where f is focal length and N is the f-number.

The focal length value is not required when comparing two lenses of the same focal length; a value of 1 can be used instead, and the other factors can be dropped as well, leaving area proportion to the reciprocal square of the f-number N.

If two cameras of different format sizes and focal lengths have the same angle of view, and the same aperture area, they gather the same amount of light from the scene.

The relative focal-plane illuminance, however, depends only on the f-number N, independent of the focal length, so is less in the camera with the larger format, longer focal length, and higher f-number.

A big (1) and a small (2) aperture

Maximum and Minimum Apertures

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The specifications for a given lens typically include the minimum and maximum apertures. These refer to the maximum and minimum f-numbers the lens can be set at to achieve, respectively.

A typical lens will have an f-number range from f/16 (small aperture) to f/2 (large aperture) (these values vary). The maximum aperture (minimum f-number) tends to be of most interest (and is always included when describing a lens).

This value is also known as the lens speed, because it is proportional to the square of accepted light, and thus inversely proportional to the square of required exposure time (i.e. using a lens with f/2, one can take pictures at one quarter of the exposure time necessary using a f/4 lens).

Professional lenses for 35mm cameras can have f-numbers as low as f/1.0, while professional lenses for some movie cameras can have f-numbers as low as f/0.75 (very large relative aperture). These are known as "fast" lenses because they allow much more light to reach the film and therefore reduce the required exposure time. Stanley Kubrick's film Barry Lyndon is notable for having scenes shot with the largest relative aperture in film history: f/0.7.

Prime lenses have a fixed focal length (FFL) and large aperture and are favored by professionals, especially by photojournalists who often work in dim light, have no opportunity to introduce supplementary lighting, and need to capture fast breaking events.

Zoom lenses typically have a maximum aperture (minimum f-number) of f/2.8 to f/6.3 through their range. A very fast zoom lens will be constant f/2.8 or f/2, which means the relative aperture will stay the same throughout the zoom range. A more typical consumer zoom will have a variable relative aperture, since it is harder and more expensive to keep the effective aperture proportional to focal length at long focal lengths; f/3.5 to f/5.6 is an example of a common variable aperture range in a consumer zoom lens.

f/32 - narrow aperture and slow shutter speed

f/5 - wide aperture and fast shutter speed