Exposure index
From Wikipedia, the free encyclopedia
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. In this case it is said that film has been shot at EI 800.
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; similarly, a light meter can be known to understate or overstate lighting conditions. In such cases one could adjust EI rating accordingly in order to compensate for these effects and consistently produce correctly exposed negatives.
