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Can someone explain guide numbers?

April 26, 2017explain guide Numbers

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Can someone explain guide numbers?

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The flash guide number for a given ISO is the *flash* distance multiplied by the f-stop. It can be converted from one ISO to another by the formula GN2=GN1(sqrt(ISO2/ISO1)). The guide number is more accurate for handle and shoe mount flashes. Studio flashes have trouble with it because of the variety of reflectors that are too different from a point source. When using guide numbers for photomacrography calculations, measure distance from the back of the hot shoe.