exposure index

Definition: EI: Setting used to calculate camera exposure settings. * EI is set on the exposure meter or camera in order to give exposures based not on actual film speed but on an adaptation or correction of it for a special purpose. * E.g. (a) ISO 400 speed film may be exposed at E.I. 800 in the expectation of being given extra development; (b) ISO 100 film may be exposed at E.I. 125 in order to compensate for its emulsion speed being faster than nominal; (c) ISO 25 film may be exposed at E.I. 12 in anticipation of fine-grain processing.

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