GN is an acronym for “Guide Number” and is used as a specification for flash power.  The higher a GN, the more powerful the flash.  A GN is usually listed in feet or meters, along with an aperture and ISO: GN of 38m (125ft) @ 35mm,  ISO 100.  What this means is that you can get a good exposure up to 125ft with an aperture of f/1. Since most people don’t have an f/1 lens, you can use the following equation: Aperture = GN/distance. Therefore:  f4 = 125’/31′.

A GN is a useful tool for comparing flashes and can help with manual flash calculations.

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