exposure factor

Definition: Number indicating degree by which measured exposure should be corrected or changed in order to give exposure corrected for use with an accessory e.g. filter, extension tube or when certain working circumstance e.g. close-up focusing, or reciprocity failure need correction. * Exposure factor increases exposure by (a) simple multiplication of exposure time e.g. factor of 2 increases exposure time from 1/500 to 1/250sec or (b) by changing the set f/number by number of stops equal to log2 of the factor i.e. a factor of 2 equals 1 stop; a factor of 4 equals 2 stops.* Exposure factors are often quoted in stops e.g. "add an extra 2 stops" means an uncorrected reading of f/16 for 1/60sec should be corrected to e.g. f/8 for 1/60sec or the equivalent.

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