Guide Number Calculator
Flash guide number: distance and aperture relationships.
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Guide number basics
Flash guide number (GN) relates aperture and distance at a stated ISO (usually ISO 100): GN = distance × f-number (metric GN uses meters).
distance = GN / N | N = GN / distance
FAQ
Does ISO change GN?
Yes. Higher ISO effectively increases usable GN. Roughly, +1 stop ISO → multiply GN by √2.
Guide numbers in mixed lighting
Manufacturer GNs assume a specific zoom head position and often a reflective room. Softboxes and umbrellas reduce effective GN dramatically. Measure or chimping after computing a starting aperture or distance. When bouncing flash, treat the path length as the sum of camera-to-ceiling and ceiling-to-subject distances, then open up beyond the bare-GN prediction.