Film speed

Uncommon card · No. 193743

In analog photography, film speed is the measure of a photographic film's sensitivity to light; in digital photography, the term describes the lightness of an image sensor's output in relation to its exposure. The measurement of film speeds is a concern of sensitometry, the scientific study of light-sensitive materials. Various numerical scales were developed to specify film speeds, with the most recent being the ISO system introduced in 1974. Prior to that, the ASA system were commonly used in the United States, and the DIN in Europe. The term speed comes from the early days of photography. Photographic emulsions that were more sensitive to light needed less time to generate an acceptable image and thus a complete exposure could be finished faster, with the subjects having to hold still for a shorter length of time. Emulsions that were less sensitive were deemed "slower" as the time to complete an exposure was much longer and often usable only for still life photography. Exposure times...

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