Flash Guide Number Calculator
Enter any two of guide number, aperture and distance to solve the third. The guide number equals the aperture times the distance, scaled for ISO.
How to use the flash guide number calculator
- Enter exactly two of guide number, aperture and distance.
- Optionally set the shooting ISO and the rated reference ISO.
- Read the solved value plus the effective guide number at your ISO.
Examples
Aperture from GN and distance
GN 56 (ISO 100), 7 m
f/8
ISO 400 doubles reach
GN 56, 7 m, ISO 400
Effective GN 112, so f/16
Frequently asked questions
What is a flash guide number?
A guide number rates flash power as the aperture times the subject distance for a correct exposure at a reference ISO. A GN of 56 in metres means f/8 at 7 metres.
How do I find the aperture from a guide number?
Divide the guide number by the flash-to-subject distance. With a GN of 56 in metres and a subject 7 metres away, the aperture is 56 divided by 7, which is f/8.
How does ISO change the guide number?
The effective guide number scales with the square root of the ISO ratio. Going from ISO 100 to ISO 400 is two stops, so the effective guide number doubles and the flash reaches twice as far.
Are guide numbers in metres or feet?
Either, but the unit must match your distance. A guide number quoted in metres needs distance in metres. The same flash has a larger guide number when quoted in feet.
Does the guide number assume full power?
Usually yes, at a stated zoom and reference ISO. Reducing flash power lowers the effective guide number, and zooming the flash head narrower raises it.
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