American film speed standard (1943–1987) defining sensitivity values like ASA 100 or 400, now adopted as ISO ratings.
Technical Details
The ASA scale is based on logarithmic measurements of film density at standardized exposure values. Standard ASA values were 25, 50, 100, 200, 400, 800, and 1600, with each step representing exactly one f-stop difference. For example, ASA 100 required an exposure of 1/125s at f/16 in daylight (5600K), while ASA 400 correctly exposed the same scene at f/32 or 1/500s at f/16. The measurement methodology followed strict laboratory guidelines with defined development times and chemical concentrations.
History & Development
In 1943, the American Standards Association introduced the ASA system to standardize the previously inconsistent sensitivity ratings of various film manufacturers. In 1974, ASA merged with the German DIN system to form the ISO standard, with the numerical ASA values remaining identical to the ISO numbers. Kodak had been using a similar system since 1942, which was seamlessly integrated into ASA. In 1987, ASA was officially replaced by ISO, although many cinematographers continued to refer to "ASA values" for decades.
Practical Application in Film
Cinematographer Gordon Willis deliberately chose ASA 50 film for "The Godfather" (1972) to create dark interiors with extreme contrast and fine grain. Documentarians preferred ASA 400 stocks like Kodak 5294 for available-light situations, while studio shoots were mostly realized with ASA 100 (Kodak 5254). For night shoots, crews opted for ASA 800 emulsions, but accepted significantly visible grain. The choice of ASA directly determined the lighting intensity: ASA 100 required approximately 2000 lux for interior shots, while ASA 400 only needed 500 lux.
Comparison & Alternatives
ASA differed from the European DIN system through arithmetic rather than logarithmic scaling – ASA 100 corresponded to 21° DIN. The Soviet GOST system also used different values. Modern digital cameras adopt the ISO designation (identical to ASA numbers) but significantly expand the range: Arri Alexa achieves ISO 3200 with less noise than ASA 400 film grain. In film emulation during post-production, colorists continue to reference the classic ASA characteristics of specific stocks.