Page "Film stock" Paragraph 28

A critical property of a stock is its film speed, determined by ASA or its sensitivity to light listed by a measurement on the raw stock which must be chosen with care.

Speed determines the range of lighting conditions under which the film can be shot, and is related to granularity and contrast, which influence the look of the image.

The stock manufacturer will usually give an exposure index ( EI ) number equal to the ASA which they recommend exposing for.

However, factors such as forced or non-standard development ( such as bleach bypass or cross processing ), compensation for filters or shutter angle, as well as intended under-and over-exposure may cause the cinematographer to actually " rate " the stock differently from the EI.

This new rating is not a change to the stock itself-it is merely a way of calculating exposure without figuring out the compensation after each light reading.