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Film speed
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Film speed

Film speed is the measure of a photographic film stock's sensitivity to light. A stock with relatively lower sensitivity requires a longer exposure and is thus called a slow film, while a stock with relatively higher sensitivity can shoot the same scene with a shorter exposure and is called a fast film.

The standard known as ISO 5800:1987 from the International Organization for Standardization (ISO) defines both a linear scale and a logarithmic scale for measuring film speed.

In the ISO linear scale, which corresponds to the older ASA scale, doubling the speed of a film (that is, halving the amount of light that is necessary to expose the film) implies doubling the numeric value that designates the film speed. In the ISO logarithmic scale, which corresponds to the older DIN scale, doubling the speed of a film implies adding 3° to the numeric value that designates the film speed. For example, a film rated ISO 200/24° is twice as sensitive as a film rated ISO 100/21°.

The most common ISO film ratings are 25/15°, 50/18°, 100/21°, 200/24°, 400/27°, 800/30°, 1600/33°, and 3200/36°. Consumer films are generally rated between 100/21° and 800/30°, inclusive.

The following table shows the correspondence between these scales:

ISO linear scale ISO logarithmic scale
same as old ASA scale same as old DIN scale
25 15°
32 16°
40 17°
50 18°
64 19°
80 20°
100 21°
125 22°
160 23°
200 24°
250 25°
320 26°
400 27°
500 28°
640 29°
800 30°
1000 31°
1250 32°
1600 33°
3200 36°

In a broad way, speed is inversely related to granularity, which is, literally, the size of the grains of silver halides in the emulsion. A fine-grain stock, such as the ones used for the intermediate stages of copying the original camera negatives, is "slow", meaning that the amount of light used to expose it must be high. Fast films, used for shooting in poor light, produce a grainy image. The image actually consists of a mosaic of developed and undeveloped areas of the emulsion, and each grain of silver halide develops in an all-or-nothing way. If the subject has an edge between light and darkness and that edge falls on a grain, the result will be an area that is all light or all shadow. An accumulation of such areas breaks up the visible contours of the object, the effect known as graininess.

Fast films are also relatively contrasty, for the same reason. That is, an area of the image will consist of bright areas and dark ones with few transitional areas of midtones.

In the early 1980s, there were some radical improvements in film stock. It became possible to shoot color film in very low light and produce a fine-grained image with a good range of midtones. In advertising, music videos, and some drama, it is fashionable to create mismatches of grain, color cast, and so forth between shots. These are deliberate; sometimes they are added in post-production.

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