Guide Numbers

Portable flash units designed to be used with small-format and medium format cameras are given a 'guide number' (GN) by the manufacturers. This denotes its potential power output. The guide number is an indication of the maximum distance at which the unit can be used from the subject to obtain an appropriate exposure. The guide number is rated in metres at 100 ISO and an aperture of F1. Because very few photographers possess an F1 lens the actual maximum working distance is usually somewhat lower than the guide number would suggest.

A unit given a guide number of 32 could correctly expose a subject at 16 metres using an aperture of F2 (32 divided by 2 = 16). If the lens was changed to one with a maximum aperture of F4, the maximum working distance would be reduced by eight metres.

It therefore follows that if the guide number of the unit is known the correct exposure can be determined by dividing the guide number by the working distance. The resulting figure is the aperture required when the flash is turned to manual full power (photocell effectively switched off). For example, a flash with a guide number of 32 used to expose a subject four metres away will require an aperture of F8.

Calculating guide number, distance and apeture.
Distance from subject x indicated aperture (MIE) = Guide number

Guide number / Aperture = Effective working distance

Guide number / Distance from subject = Correct aperture