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.
This film container denotes its speed as ISO 100/21°, including both arithmetic (100 ASA) and logarithmic (21 DIN) components. The second is often dropped, making (e.g.) "ISO 100" effectively equivalent to the older ASA speed. (As is common, the "100" in the film name alludes to its ISO rating.)
A box of Agfacolor Neu with the instruction "expose as 15/10° DIN" (in German).
Weston Model 650 light meter from about 1935
Early Weston Master light meter 1935-1945
Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity, contrast, and resolution of the film. Film is typically segmented in frames, that give rise to separate photographs.
Undeveloped 35 mm film roll
A roll of 400 speed Kodak 35 mm film
A Polaroid instant photograph
135 Film Cartridge with DX barcode (top) and DX CAS code on the black and white grid below the barcode. The CAS code shows the ISO, number of exposures, exposure latitude (+3/−1 for print film).
