Exposure and shooting
ISO (film speed)
A number that says how sensitive a film is to light: the higher it is, the less light the film needs.
Doubling the number means the film needs half the light, a difference of one stop. Higher-speed films generally show more grain, and the number printed on the box is the film’s box speed. ISO speeds are standardised so that films from different makers can be compared, but you are free to rate a film differently when you shoot it.