guide number

Definition: Value used in flash photography to calculate the correct lens aperture for a given flash-to-subject distance, flash coverage and film speed. * Guide number for a given film speed, when divided by distance (metric distance for metric guide number), equals lens aperture in f/numbers. * E.g. for ISO 100 film, a metric guide number of 45 and a distance of 5m gives a working aperture of 45 divided by 5 equals f/9 (for given flash coverage). * Alternatively, should you wish to set f/number, then Guide Number divided by f/number equals distance. * Also known as guide factor.

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