Film Speed
A film's sensitivity to light, rated on the ISO scale and formerly as ASA or DIN.
Formula / rule
ISO arithmetic = 2^((DIN−1)/10)
Measured in
ISO
In depth
Unlike a digital ISO setting, film speed is a fixed physical property of the emulsion — the whole roll shares it. Doubling the number halves the exposure required. The rating is determined by a standardised test at a specific development, which is why rating a film differently and adjusting development is a legitimate technique rather than an error.
Examples in practice
ISO 400 film needs half the light of ISO 200 film for the same density on the negative.