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 log scale	example of film stock
(old ASA scale)	(old DIN scale)	with this nominal speed
6	9°
8	10°
10	11°
12	12°
16	13°
20	14°
25	15°	old Agfacolor, Kodachrome 25
32	16°
40	17°	Kodachrome 40 (movie)
50	18°	Fuji RVP (Velvia)
64	19°	Kodachrome 64
80	20°	Ilford Commercial Ortho
100	21°	Kodacolor Gold
125	22°	Ilford FP4
160	23°	Fuji NPS
200	24°	Fujicolor Superia 200
250	25°
320	26°	Kodak Tri-X Pan (TXP)
400	27°	Kodak T-Max (TMX)
500	28°
640	29°
800	30°	Fuji NPZ
1000	31°
1250	32°
1600	33°	Fujicolor 1600
2000	34°
2500	35°
3200	36°	Ilford Delta-3200
4000	37°
5000	38°
6400	39°

A film speed is converted from the linear scale to the logarithmic scale by this formula (plus rounding to the nearest integer):

<math>\textit{log-scale~speed} = 21 + 3\,\log_2 \frac{\textit{linear~speed}}{100}<math>

Conversion from the logarithmic scale to the linear scale is analogous, except that results must be rounded to the conventional values of the linear scale listed in the table above.

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.

References

• Leslie Stroebel, John Compton, Ira Current, and Richard Zakia. Basic Photographic Materials and Processes, second edition. Boston: Focal Press, 2000. ISBN 0-240-80405-8.

External link

• Purchase the official ISO 5800:1987 (http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=11948).

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