ABSTRACT
Film speed is a standard measure of its sensitivity to light. The International Organization for Standardization sets the standard tests for film speed that give us the familiar ISO numbers. Prior to adopting the ISO system, arithmetic ASA numbers were used in America and Japan while logarithmic DIN numbers were largely used in Europe – both numbers live on in the full ISO number, such as ISO 100/21° (arithmetic/logarithmic). Photographers tend to use only the arithmetic value where double the number means double the speed – one stop faster. Film is commonly available from ISO 25 up to ISO 3200. Many modern film cameras have electrical contacts in the film chamber to read the film speed from the patches of bare metal on the film canister. The film speed is encoded in the form of a primitive bar code. Digital camera sensitivity is an exact equivalence of the film ISO rating.
