Guide Number (GN)
A number that is shown on a flash unit that indicates its ability to illuminate a subject at a specific distance while using a particular ISO level. In most cases, the guide number is calculated by multiplying the distance between the flash and the subject by the f-stop setting that is required to get an accurate exposure of the subject at that distance. This calculation is performed based on an ISO setting of 100. When compared to a flash that has a higher guide number, one that has a lower guide number generates a significantly less powerful flash.
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