Film speed

Film 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 the mid-1970s. A closely related ISO system is used to describe the relationship between exposure and output image lightness in digital cameras. Prior to ISO gaining traction the best known systems were ASA in the U.S. and 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 their pose and not make any body movements for a shorter length of time. Emulsions that were less sensitive were called slower because the time to complete an exposure was much longer and often usable only for still lifes. Exposure times for early photographic emulsions went from hours to minutes and then seconds and fractions of a second in the 1800's.

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 of other types). In short, the higher the sensitivity, the grainier the image will be. Ultimately sensitivity is limited by the quantum efficiency of the film or sensor.

To determine the exposure time needed for a given film the light meter was usually used.