Film Speed.

A colloquial term for the exposure index. This is, in this context, the light sensitivity of film, rated according to an internationally defined standard.

There are a number of different historical standards for film speed, but the universal one in use today is the ISO film standard, which derives from the older ASA film standard.

The term “speed” refers to the speed at which the film emulsion reacts to the presence of light. Fast film is very sensitive to light - it reacts rapidly and so can be used in low-light conditions or with fast shutter speeds. Slow film is not as sensitive and so reacts more slowly.

There’s a complex relationship between film grain (resolution, in a sense) and film speed. All things being equal, faster film is grainier than slower film. This is because the speed at which film emulsion reacts to light is related to the physical size of the silver halide grains in the film. Larger clumps react more quickly but have greater grain size.

However, the past two decades or so have seen tremendous improvement in film grain quality and so even ISO 800 speed film today has quite fine grain. Not as fine as ISO 64 film, of course, but more than adequate for a lot of work, especially if the final image isn’t going to be dramatically enlarged.