exposure index (EI)

A number in an arithmetical sequence used to specify the sensitivity of photographic film in actual use. This number may differ from the film speed printed on the box using the ISO, ASA, or DIN speed systems. It can represent a preferred film speed rating, or one that is necessary when a filter is used or when the film is uprated for ‘push processing’ to increase the effective film speed.

Exposure index, or EI, in photography refers to film speed rating assigned to a particular film by a photographer in order to compensate for equipment calibration inaccuracies, process variables, or to achieve certain effects. Exposure index may or may not be the same as manufacturer's film speed rating for that particular film.

For example, a photographer may choose to rate a 400 ISO speed film at 800 and then use push processing in order to get printable negatives from low-light conditions.

Another example of a situation when exposure index would differ from film manufacturer's rating is when a camera shutter is known to be miscalibrated and consistently overexposes or underexposes the film;