Flash Guide Number
Relates flash power, aperture and subject distance.
Formula
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Example
GN=40, subject 5 m: N = 40/5 = f/8
How it works
The guide number is a compact way to describe how much light a flash can deliver at a given ISO setting, combining the effect of flash output and distance into one number. Because light spreads out as it travels, the farther the subject is, the larger the aperture must be to collect enough light; multiplying the f-number by the subject distance gives the guide number for a correctly exposed shot.
When to use it
Use this when you want to estimate a flash aperture quickly, especially with manual flash or older speedlights that list a guide number. It is useful for portraits, event snapshots, or any situation where the flash-to-subject distance is known and constant.
Worked example, step by step
A flash has a guide number of 56, and the subject is 7 m away. Use GN = N × d Rearrange to find the aperture: N = GN ÷ d N = 56 ÷ 7 N = 8 So the correct aperture is f/8.
Common mistakes to avoid
- Forgetting to match the guide number to the same ISO used for the shot
- Using feet and metres together instead of one consistent distance unit
- Rearranging the formula incorrectly and dividing the distance by the guide number
Did You Know?
Guide numbers assume ISO 100; doubling ISO extends a flash's reach by about 40%.
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