ASA ISO DIN Equivalence

ASA, ISO and DIN: Film Speed Standards

Film speed, or photographic sensitivity, tells you how strongly an emulsion reacts to light. Three old standards are still floating around. ASA came from the American Standards Association, ISO (ISO 5800) is the modern international one, and DIN is the Deutsche Industrie Norm from Germany. ASA and ISO run on an arithmetic scale and give the same numbers, so ISO 100 is ASA 100 and ISO 400 is ASA 400. Double the figure and you've gained a full stop of sensitivity, meaning the film is twice as fast.

DIN works differently. It's a logarithmic scale, and every +3° is worth one stop. The conversion is DIN = 10 × log₁₀(ISO) + 1. In practice that gives you ISO 100 = DIN 21°, ISO 200 = DIN 24°, ISO 400 = DIN 27°, ISO 800 = DIN 30°, ISO 1600 = DIN 33°. The same film stock would say "27 DIN" on an old European box and "ASA 400" on an American one.

Applications

This comes in handy with vintage cameras like the Leica, Rolleiflex or Zeiss Ikon that meter only in DIN, when reading old light-meter charts, or when decoding film boxes from the 1960–1980s. It also helps if you're calibrating an analog camera whose ISO dial shows DIN alone, or shooting expired film where the box reads "21/100°" in the combined ISO/DIN notation introduced in 1987.

FAQ

Is ASA still used? Not anymore. ISO took over in 1982. Since the numbers match exactly, a Kodak Tri-X 400 ASA is the same film as Tri-X ISO 400.

Why does DIN have a degree symbol? The ° sign is there to keep it apart from the arithmetic ASA/ISO number. You read "21°" as "twenty-one DIN degrees".

What about GOST? The Soviet GOST scale runs close to ASA but sits a little off, so GOST 90 ≈ ASA 100. We don't handle it here, since it was phased out after 1987.