A number used to represent the illuminating power of a photographic flash.
When electronic flash is used, the camera shutter opens, the flash fires, and the shutter closes, but because the flash duration is very short, the shutter has no control over the amount of light falling on the film. Consequently, the level of film exposure due to the flash can only be controlled by the lens aperture setting.
In metric countries, the flash guide number is the aperture setting required for ISO 100 film when the flash to subject distance is 1m (in North America, the guide number is the 100ASA aperture setting for a distance of 1 foot. Divide the American guide number by 3.28 to get it in meters).
Using the Guide number G, the required aperture setting for any film speed and distance (in air) is obtained thus.
Normally, this equation is used to produce a guide table, which is attached to the flash.