Tuesday, June 10, 2008

Maximum and Minimum Apertures

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

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