Film Speed

Film speed is the term often used to describe an emulsion's sensitivity to a given amount of light.

Sensitivity, in turn, is a product of the amount and configuration of silver halide crystals coated on the film's emulsion layer. This is true even for color and chromogenic black-and-white films where the final result is devoid of silver, but instead is composed of dye clouds. Even these two film types begin with the exposure of light-sensitive silver,

The difference between ISO and DIN

There are two type of scales being used to denote film speed.

  1. ISO which stands for International Standards Association, the body setting many industrial standards for precision measurement.

  2. DIN (a German designation meaning Deutches Industrie Norm) serves the same purpose as ISO does for the remainder of the photographic world.

The two scales, ISO and DIN, are different in one respect.

  1. ISO is arithmetic; the doubling of an ISO number indicates an exact doubling of film speed or sensitivity (e.g., ISO-200 film is twice as sensitive as an ISO-100 emulsion).

  2. The DIN system, on the other hand, is not an arithmetic progression but is instead logarithmic. On the DIN scale, moving up three numbers denotes a doubling of film speed (e.g., going from DIN 18 to DIN 21 denotes doubling film speed). The equivalent ISO numbers are ISO 50 to ISO 100.

Film speed is also denoted by a commonly accepted verbal classification system:
Slow ISO 6-50 DIN 9-18
Medium ISO ISO 64-160  DIN 19-23
Fast ISO ISO 200-800 DIN 24-30
Ultra-Fast ISO ISO 1000-3200 DIN 31-36

In addition to the intensity of the light striking a film's surface, there are several other factors affecting film speed. Many black-and-white films are not sensitive to all portions of the spectrum equally and because of this have different EIs. As an example, if you would choose to use a black-and-white film whose red sensitivity was low indoors under red-rich tungsten illumination, the film's effective speed will be lower than its rated speed. In daylight, or using electronic flash (which is not as red-rich), the normal speed rating will be restored.
Both black-and-white and a number of color films' speed ratings can be successfully uprated to provide more than normal sensitivity. An ISO-100 film can be exposed at ISO 200 or more, often with little sacrifice in picture quality.

More about film photography

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