The sensitivity of the film. The amount of light which strikes the film. The length of time to which the film is exposed to light. Proper exposure requires that the correct volume of light strikes the film for the right amount of time, as required according to the designated sensitivity of the film. These fine-grained films are a good choice for large enlargements because the grain pattern does not show up in the finished photographs.
Fast films have larger clumps of grain and therefore are more sensitive to light than slow films.
Fast films are a good choice for low-light situations or action photos where a fast shutter speed is required. However, when enlarged the grain pattern may appear on the finished picture. The amount of light which strikes the film is determined by the diameter of the opening in the lens. A large aperture lets in a large quantity of light and a small aperture lets in small amounts. This relationship is usually engraved on the lens as a reciprocal.