Guide number

A measure of the power of a flash.

Guide number is equal to the maximum distance of the object being photographed multiplied by the lens aperture f-number, which still gives correct illumination of the subject.

Guide numbers have units of metres or feet, depending on the units used to measure the distance. The guide number will vary with film/sensor sensitivity, so should be quoted with the ISO number - "GN 38m (ISO 50)".

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