, the ISO have defined the standard for measuring film speed
. The ISO rating of a film
gives a (relative) indication of how quickly the emulsion
on the surface of the film
reacts to light.
A film's ISO rating is a combination of it's ASA rating and the equivalent DIN rating. For example, an ISO 200/24° rating indicates a film speed rated at 200 ASA/24° DIN. In reality, the DIN rating is less commonly used, (probably because it uses a logarithmic scale), so the ASA and ISO ratings are effectively interchangeable.
The higher the ISO number, the "faster" the film is said to be. A doubling in ISO speed indicates twice the light sensitivity.