Sensitivity to light determines film speed ratings. Faster films are able to attain correct exposure in less time than slower films. The speed rating system, maintained by the International Standards Organization (ISO), is most commonly expressed in multiples of 100 (100, 200, 400, 1000), though there are other values available, such as 25 and 50. With each doubling of the film speed rating, its sensitivity to light is also doubled. A film rated at ISO 200 requires half the exposure time as one at ISO 100, but twice as much time as an ISO 400 film (assuming all other values, such as ambient light and the camera's aperture setting remain constant).

Faster films are more versatile than slower films, because they can be used in low light conditions where a flash is impractical. They are also ideal for capturing action shots, where you need a fast shutter speed to freeze the action (such as in a swimmer's dive or an auto race).

Slower films have a finer grain and are therefore capable of producing photos of greater detail. These films are ideal for indoor portraits where a flash will be used, or for shooting outdoors in bright sunlight. Enlargements can be made with less quality loss than would be the case with a faster film, which tends to look grainy with enlargement.