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gn

Used to express the power output of the flash nit. It indicates the power ofa flash in relation to ISO film speed. Guide numbers are quoted in either metersor feet. Guide numbers are used to calculate the f/stop for correct exposure as follows:Number calculated by multiplying proper flash exposure aperture by the subject distance.

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gn

Abbreviation for Guide Number.
Photo Tips

gn

With distance-priority manual flash operation the guide number is automatically determined by the Speedlight according to the distance value and aperture set.
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