Slow films need much more light to form a usable image than do fast films, which are highly sensitive. This means that you can more easily take pictures in dim light with fast film. In brighter light, fast film allows you to select a fast shutter speed or a small aperture if needed. Slow films have smaller grains and can record finer detail, but unless the light is bright, they may force the photographer to use an unsuitably slow shutter speed or too wide an aperture. In average daylight, films of medium or medium-fast speed offer a good compromise.
Fast films are an advantage in poor light or for action photographs requiring fast shutter speeds.
Slow films are useful for static, detailed subjects, such as still-life or architecture. Each doubling or halving of the ISO number indicates a halving or doubling of speed, changing the exposure required by one full setting on either the aperture or shutter speed scales. The film speed rating and other details are clearly marked on the box. You set the camera to the film speed by lining up the corresponding number on the rewind knob or the shutter speed dial. A tripod is highly recommended.