Guide number (GN).
A number describing the coverage distance or maximum range of an electronic flash unit. The guide number is equivalent to the current f stop of the camera multiplied by the coverage distance.
f-stop number = GN / distance
distance = GN / fstop numberGuide numbers are important in manual flash calculations. Electronic flash units are often sold on the basis of their guide numbers. Note a few points about this, however.
First, since the guide number incorporates a distance value it’s in either metric (metres) or Imperial (feet). Be absolutely certain when comparing guide numbers that you have the unit correct. Second, the lens aperture is part of the guide number. Most flash unit manufacturers seem to define their GNs using a lens aperture of f/1.4, but that is by no means guaranteed. Fourth, it’s a “guide” number, not an absolute power output value or anything. Each manufacturer has its own idea as to what constitutes acceptable exposure for the distance range, and is usually excessively optimistic as to the output of the flash unit. Finally, the guide number is just used in flash calculations to determine the appropriate aperture required to cover a certain distance or vice-versa. In other words, the guide number describes the distance coverage of a flash, not its actual power output as such. Because of the inverse square law of light falloff, a flash unit has to have four times the power output in order to throw light twice as far.
For more information please consult the guide number section of my flash document.
cf. aperture, flash unit, inverse square law.
Entry last updated 2002-04-05. Term 578 of 1487.
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