To determine the exposure time needed for a given film, a light meter is typically used.įilm speed measurement systems Historical systems Warnerke Ultimately sensitivity is limited by the quantum efficiency of the film or sensor. Generally, the higher the sensitivity, the grainier the image will be. In both film and digital photography, the use of higher sensitivities generally leads to reduced image quality (via coarser film grain or higher image noise). Exposure times for photographic emulsions shortened from hours to fractions of a second by the late 19th century. 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. 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. The term speed comes from the early days of photography. Prior to ISO, the most common systems were ASA in the U.S. A closely related system, also known as ISO, is used to describe the relationship between exposure and output image lightness in digital cameras. (As is common, the "100" in the film name alludes to its ISO rating.)įilm speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system introduced in 1974. The second is often dropped, making (e.g.) "ISO 100" effectively equivalent to the older ASA speed. This film container denotes its speed as ISO 100/21°, including both arithmetic (100 ASA) and logarithmic (21 DIN) components.
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