ABSTRACT
Two films are said to differ in speed if the exposure required to produce a negative on one differs from the exposure required to produce a negative of similar quality on the other, the material requiring the lower exposure being said to have the higher speed. The speed of a material thus varies inversely with the exposure required, and we can therefore express speed numerically by selecting a number related to exposure. The response of the photographic emulsion is complex, and speed depends on many factors, of which the following are the most important: Exposure
The colour of the exposing light, e.g. whether daylight or tungsten light. The intensity of the exposing light. This is because of reciprocity law failure (Chapter 15).
