A flash's guide number (GN) ties together distance and aperture: GN = distance × f-number at ISO 100. Enter your flash's GN (in meters), ISO, aperture and subject distance.
Power settings below full reduce GN by √2 per stop.
How it works
Effective GN = GN × √(ISO/100) ÷ √2^(power stops). Distance = GN ÷ N; aperture = GN ÷ distance.
FAQ
Is GN measured at a specific zoom?
Yes — manufacturers often quote GN at the flash head's longest zoom setting. Check the spec sheet.
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