Flash Guide Number Calculator
Flash Distance from GN
Calculate maximum flash distance
Required Aperture
Calculate aperture for a given flash distance
Flash Power Setting
Calculate the guide number at reduced power
Formula
GN = Distance × f-stop | Distance = GN / f-stop | Effective GN = GN × √(ISO/100) | Reduced GN = GN × √(power fraction)
Frequently Asked Questions
What is a flash guide number?
A guide number (GN) indicates flash power. It equals the maximum distance times the f-stop at ISO 100. GN = Distance × Aperture. A GN of 58 means the flash can illuminate a subject 10.4m away at f/5.6.
How does ISO affect flash range?
Doubling the ISO increases effective flash range by 1.4x (square root of 2). Going from ISO 100 to 400 doubles the effective guide number, doubling the range.
What does reducing flash power do?
Each halving of flash power reduces the guide number by 0.7x (1 stop). This reduces range but speeds up recycle time and helps balance flash with ambient light.
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