Guide Number
A measure of a flash's power, equal to the product of the correct aperture and the subject distance at a stated ISO.
Formula / rule
GN = N × d
Measured in
m at ISO 100
In depth
Divide the guide number by the distance to get the aperture, or by the aperture to get the distance. It is a rough guide at best on modern flashes because the figure varies with the zoom head position — a unit quoted at GN 60 may be quoting its 200 mm setting, and will manage far less at 24 mm.
Examples in practice
A flash with GN 36 at ISO 100 correctly exposes a subject at 4.5 m when set to f/8.