Photography & Imaging
Intermediate

Flash Guide Number

Relates flash power, aperture and subject distance.

Formula

GN=N×dGN = N \times d

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Variables

GNGuide number
NF-number
dDistance (m)

Example

GN=40, subject 5 m: N = 40/5 = f/8

Did You Know?

Guide numbers assume ISO 100; doubling ISO extends a flash's reach by about 40%.

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