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Flash Shooting Distance

Calculate the ideal shooting distance to your subject from a flash unit's guide number, aperture and ISO.

Input

Work out the ideal shooting distance to your subject from a flash unit's guide number (GN), aperture (f-stop) and ISO. Enter the GN measured at ISO 100 in meters.

GN

Aperture (f-stop)

F
ISO sensitivity

Result

Ideal shooting distance

5.0m

GN28.0 × √(ISO100/100) ÷ F5.6

Effective guide number

GN 28.0

Aperture

F5.6

ISO sensitivity

ISO 100

Ideal distance by aperture (effective GN ÷ f-stop)

Aperture (f-stop)Ideal distance
F1.420.0 m
F214.0 m
F2.810.0 m
F47.0 m
F5.6 (your input)5.0 m
F83.5 m
F112.5 m
F161.8 m

How it works

  • The ideal shooting distance is found from 'guide number ÷ aperture (f-stop)'. Enter the guide number (GN) measured at ISO 100 in meters.
  • Changing the ISO also changes how far the light reaches. This tool applies the sensitivity gain to the GN as 'effective GN = GN × √(ISO ÷ 100)', then divides that effective GN by the aperture to find the distance.
  • Raising the ISO fourfold (e.g. 100 to 400) doubles the effective GN, extending the ideal distance roughly twofold. Conversely, stopping down (a larger f-number) shortens the reach of the light.
  • Beyond the distance for the aperture you entered, the result panel also lists the ideal distance for each f-stop from F1.4 to F16. Use it as a guide when picking an aperture to suit your composition.
  • These values are theoretical, based on the manufacturer's rated guide number (direct light under standard reflection conditions). Actual ideal distance varies with wall or ceiling bounce, diffuser use, zoom head position and more, so confirm with a test shot.