Flash Distance Calculator
Working distance from GN and f-number.
Inputs
Results
Flash falloff
Light follows the inverse square law: double the distance, illumination drops to about one quarter (−2 stops). Guide numbers encode that relationship for a given power and head zoom.
FAQ
Why is the subject underexposed at the calculated distance?
Modifiers (softboxes, umbrellas), zoom position, battery level and color of walls all change effective output. Use GN as a starting point and meter or chimping to refine.
Distance and modifiers
As you back a strobe away, inverse-square falloff darkens the subject quickly. The calculator assumes bare-GN behavior; grids and snoots waste light and need more power or closer placement. For group photos, place the key light so the nearest and farthest faces stay within about one stop—check distances here, then meter the real stand-ins.