An ISO number rating refers to the speed of the chosen film. A lower ISO film, such as 100 speed film, will require more light. A higher ISO film, such as 400 speed film, will require less light, but produce grainier negatives. The higher the ISO, the less light required. Typically, lower speed films are used for portraits, whereas higher speed films are used in lower light settings or in sports photography where one wishes to stop action.