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. Ultimately sensitivity is limited by the quantum efficiency of the film or sensor. In short, the higher the sensitivity, the grainier the image will be. In both digital and film photography, the reduction of exposure corresponding to use of higher sensitivities generally leads to reduced image quality (via coarser film grain or higher image noise of other types). Highly sensitive films are correspondingly termed fast films. Relatively insensitive film, with a correspondingly lower speed index, requires more exposure to light to produce the same image density as a more sensitive film, and is thus commonly termed a slow film.
A closely related ISO system is used to describe the relationship between exposure and output image lightness in digital cameras. For the genre of films, see slow cinema.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.
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