Editing Film speed (section)

===Exposure index{{anchor|EI}}===
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Exposure index, or EI, refers to speed rating assigned to a particular film and shooting situation in variance to the film's actual speed. It is used to compensate for equipment calibration inaccuracies or process variables, or to achieve certain effects. The exposure index may simply be called the ''speed setting'', as compared to the speed ''rating''.

For example, a photographer may rate an ISO&nbsp;400 film at EI&nbsp;800 and then use [[push processing]] to obtain printable negatives in low-light conditions. The film has been exposed at EI&nbsp;800.

Another example occurs where a camera's [[shutter (photography)|shutter]] is miscalibrated and consistently overexposes or underexposes the film; similarly, a [[light meter]] may be inaccurate. One may adjust the EI setting accordingly in order to compensate for these defects and consistently produce correctly exposed negatives.