Manual flash · no TTL required

Flash Guide Number Calculator

Enter your flash's guide number and your film speed. Every aperture below shows the exact flash-to-subject distance that gives a correct exposure. Works with any manual flash, any camera, on film or digital.

Flash

Your flash's guide number is in its manual, quoted at ISO 100. It changes with the zoom head — use the figure for the focal length you are actually shooting.

Units
Power

Each stop down the power scale costs one stop of reach.

Film / Sensor

72 Effective guide number

Flash-to-subject distance

ApertureDistanceReach

Reverse — the distance is fixed

You are standing where you are standing. Enter the distance to your subject and read off the aperture that exposes it.

f/22

How the numbers are made

distance = GN × √(ISO ÷ 100) × √(power fraction) ÷ f-number

Guide numbers are quoted at ISO 100. Doubling the ISO multiplies reach by √2 — one stop of light, but only 1.41× the distance. Four times the ISO doubles the distance.

Modifiers cost reach. A diffusion dome typically eats 1–2 stops; bouncing off a ceiling can cost 2–3. Halve the GN for every stop lost, then read the table again.

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