The film speed of a sensitized material is a numerical expression of its sensitivity to light. The speed is measured by giving samples of the sensitized material a range of exposures under standard conditions of illumination. The amount of blackening produced after carefully controlled development is measured and plotted (usually logarithmically) against the exposure.
The speed of the material and much other information can be derived from the resulting graph, which is known as the characteristic curve. The data obtained from the characteristic curve have to be translated into a speed figure to state the speed of the material tested. This is done by means of a speed formula, which defines the speed as a function of the exposure at a specific point of the characteristic curve. The choice of such a point has been the subject of a great deal of argument in the past, since various speed systems used to base their criterion on what were to some extent arbitrary considerations.
The result is that we have a number of different film speed numbers to rate a films sensitivity to light. Some are: ASA rating, DIN, ISO Rating and BS rating. The rating is used to determine the best exposure levels and development times for the particular film.
1978, Focal Encyclopedia of Photography, Desk Edition Reprint, Focal Press, London, New York