Flikipedia A branching dictionary of the moving image

film speed

noun

A measure of a photographic emulsion's sensitivity to light, expressed on the ISO scale, that sets how much exposure the film requires to record a usable image.

Origin A compound of film and speed: a faster film reaches a given density in less time or light, by analogy with a faster process, so the more sensitive emulsion is called the faster one.

  1. Photochemistry The sensitivity of a photographic emulsion to light, quantified by a standardized number so that a fixed exposure yields a predictable image density; a higher number denotes a more sensitive, or faster, film. An ISO 400 film is one stop faster than an ISO 200 film, requiring half the exposure for the same result.

  2. Digital By extension, the equivalent sensitivity setting of a digital image sensor, also stated as an ISO value so that meters and exposure calculations carry over from film.

Definition and scale

Film speed measures how strongly a photographic emulsion responds to light. It is reported as an ISO number derived from the exposure needed to reach a specified density above the film’s base fog. The scale is arithmetic: doubling the ISO number doubles sensitivity, so an ISO 200 film needs half the light of an ISO 100 film to produce the same image, a difference of one stop of exposure.

The modern ISO figure consolidates two older systems. The American ASA standard was arithmetic and matches the arithmetic part of an ISO rating (for example, ASA 100 equals ISO 100). The German DIN standard was logarithmic, adding three units for each doubling of speed; a full ISO designation such as “100/21°” preserves both the arithmetic and the logarithmic value.

Fast and slow film

A fast film carries a high ISO number and requires little light, suiting dim scenes and brief shutter times. A slow film carries a low ISO number and requires more light. Film speed is governed largely by the size of the silver-halide crystals in the emulsion. Larger crystals intercept light more readily, raising speed, but they render as coarser grain and resolve less fine detail. Smaller crystals yield finer grain and higher resolving power at the cost of sensitivity. Film speed therefore sits at the center of a trade-off among speed, grain, and resolving power, and it interacts with a film’s dynamic range, the span of brightness the emulsion can record.

Push and pull processing

The rated speed can be altered in development. Pushing exposes the film as if it were faster than its nominal rating and compensates with extended or more active development; this permits shooting in less light but increases grain and contrast and can lose shadow detail. Pulling does the reverse, exposing as if the film were slower and shortening development, which lowers contrast. Both techniques shift the film’s effective speed away from the manufacturer’s rating rather than changing the emulsion itself.

Relation to digital sensors

A digital image sensor has no emulsion, but its amplified output is calibrated to the same ISO scale so that light meters, aperture, and shutter settings transfer directly from film practice. Raising a sensor’s ISO setting amplifies the signal from each photosite rather than altering a physical sensitivity; as with fast film, higher settings admit dimmer scenes but introduce more noise, the electronic analog of grain.

See also

  • photographic emulsion— The light-sensitive layer of film: microscopic silver-halide crystals suspended in gelatin, in which exposure forms a latent image.
  • exposure— The quantity of light reaching the film or sensor while an image is recorded, and the act of admitting it — governed jointly by aperture, shutter, and sensitivity.
  • photographic film— A flexible strip coated with a light-sensitive emulsion that records an image as a pattern of developed silver or dye.
  • dynamic range— The span of light intensities a recording medium can register between its noise floor in the shadows and its point of saturation in the highlights, conventionally measured in photographic stops.

Sources

  1. ISO 5800, “Photography — Colour negative films for still photography — Determination of ISO speed”
  2. ISO 6, “Photography — Black-and-white pictorial still camera negative film/process systems — Determination of ISO speed”
  3. Mees & James, “The Theory of the Photographic Process” (4th ed.)

Last revised: September 18, 2026