Flash Guide Number Calculator (f = GN ÷ distance)

Understanding the flash guide number calculator — concept and principle

A flash guide number (GN) describes how much light a flash puts out, expressed as the product of aperture and subject distance that gives correct exposure at full power. Photographers use it to choose the f-number for manual flash, to check how far a flash can reach at a given aperture, and to compare flash units on equal terms.

How it is calculated

• Effective GN: GN_eff = GN × √(ISO ÷ 100). The input GN is in metres at ISO 100.
• Aperture: f = GN_eff ÷ distance, with distance in metres. The result is rounded to one decimal place.
• Maximum distance at a chosen aperture: distance = GN_eff ÷ f. The calculator shows it for f/2.8 and f/8.

If your flash lists its GN in feet, divide by 3.28 to get metres first, or keep everything in feet consistently.

The principle

Light from a small source spreads over a sphere, so the illuminance on the subject falls with the square of distance (the inverse square law). The f-number controls light through the lens with the same square relationship: the lens opening area is proportional to 1 ÷ f². Both squares cancel, so a flash that correctly exposes a subject at f/8 and 4 m also exposes correctly at f/4 and 8 m. The product f × distance therefore stays constant, and that constant is the guide number. Doubling the ISO doubles sensitivity, which is one stop; one stop in f-number is a factor of √2, so the GN rises by √2 (about 1.41) and quadrupling ISO doubles it.

Worked example

1. Inputs: GN 36 (m, ISO 100), subject distance 4 m, ISO 100.
2. Effective GN: 36 × √(100 ÷ 100) = 36.
3. Aperture: 36 ÷ 4 = 9.0, so f/9.
4. Maximum distance at f/2.8: 36 ÷ 2.8 = 12.86, shown as 12.9 m.
5. Maximum distance at f/8: 36 ÷ 8 = 4.5 m.

How to read the result

f/9 is a third-stop value between f/8 and f/10, so set f/9 if your camera uses third stops, or f/8 and accept about a third of a stop brighter. A higher GN means a more powerful flash: small built-in flashes are often around GN 10 to 12, while full-size hot-shoe units commonly quote GN 36 to 60, usually measured at a telephoto zoom-head setting. At shorter distances, stop down or reduce power.

Common mistakes

• Mixing a GN in feet with a distance in metres.
• Using the GN quoted at a 105 mm or 200 mm zoom-head setting when the head is set wide, which spreads the light and lowers the real GN.
• Applying the GN to bounce flash: the light travels farther and the ceiling absorbs some of it.
• Doubling the GN for double ISO instead of multiplying by √2.

Frequently asked questions

Why does my flash GN change with zoom setting?

A narrow zoom head concentrates the same light into a smaller area, so the subject gets more light and the GN rises. Manufacturers often quote the GN at the longest zoom setting.

What does 1/2 power do to the guide number?

Half the light is one stop less, so the GN falls by √2: GN 36 becomes about 25.5 at half power.

Sources: Ralph E. Jacobson, Sidney F. Ray, Geoffrey G. Attridge, Norman R. Axford, The Manual of Photography, Focal Press