A quality of photographic film related to the shutter speed of the camera; in other words, how quickly an image will register on the film. Film speed is commonly measured according to the ASA exposure time index, a set of whole numbers which refer to a particular fraction of a second. For example, a film speed of 800 can be said to refer to a shutter speed of 1/800th of a second. Film speed is an important consideration when dealing with either fast motion (as in sports photography) or when levels of illumination are low. The less light there is, the faster a film should be to register an image. However, there is a tradeoff with fast film. The higher the film speed, the larger the grains of silver in the film, and thus the grainier the resulting photograph will be. Very fast speed (speeds over ASA 500 or so) tend to produce the grainiest images and will consequently not enlarge or reproduce well. Slower speeds have much finer grains which are less perceptible upon development. See Photography.