Monday, June 30, 2008

Photo Filter - Materials & Construction

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.

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