Фотофильмнің жарыққа сезімталдығының өлшемі

Measure of a photographic film's sensitivity to light

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 1974. A closely related system, also known as ISO, is used to describe the relationship between exposure and output image lightness in digital cameras. Prior to ISO, the most common 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 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 for photographic emulsions shortened from hours to fractions of a second by the late 19th century. 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). Generally, 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, a light meter is typically used.

Five criteria for the rating of emulsion speed have been used since the late 19th century, listed here by name and date, these criteria are: threshold (1880), inertia (1890), fixed density (1934), minimum useful gradient (1939) and fractional gradient (1939).

The threshold criterion is the point on the characteristic curve corresponding to just perceptible density above fog.

The inertia speed point of an emulsion is determined on the Hurter and Driffield characteristic curve by the intercept between the gradient of the straight line part of the curve and the line representing the base + fog (B+F) on the density axis.

The fixed density speed point is determined by defining a fixed minimum density as the basis the emulsion speed (e. g. 0.1 above B+F).

The minimum useful gradient criterion places the speed point where the gradient first reaches an agreed value (e. g. tan 𝜃 = 0.2).

The fractional gradient is defined as the speed point at which the slope of the characteristic curve first reaches a fixed fraction (e. g. 0.3) of the average gradient over a range (e. g. 1.5) of the characteristic curve.