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A guide number (GN) indicates the strength (power and distance capability) of the flash of a flash bulb or electronic flash. GNs are usually given for ISO 100 and used mainly for hot shoe mounted direct flash (as you need to know the exact distance from flash to subject). A higher guide number indicates a more powerful flash.
Exposure is affected by the light generated by the flash, so one has to take into account both the intensity of the flash and the distance from the flash to the subject. Calculate the correct aperture setting by dividing the guide number by the measured distance from the flash to subject (read from the distance scale on the rangefinder’s focusing ring once focused).
Aperture = GN / Distance to Subject (ft)
For example, the GN of the Minolta 20 flash unit (standard hot shoe connection designed for the Hi-Matics) with ISO 100 is 64ft. So for a subject at 4ft the aperture would be f/16, for a subject at 8ft the aperture would be f/8 etc. Using the QL17 G-III with the dedicated Canolite D the aperture is set automatically according to the distance of the subject in focus.
Shutter speed is assumed to be the ‘sync speed’ (the fastest shutter speed you can use with flash) for the particular camera you are using (usually 1/60 or 1/125 with an SLR but any speed with a leaf shutter camera, the faster the better if you plan on freezing motion).
GNs are fallible as the actual range of the flash will change according to the environment and the ambient light. A small living room with white walls and ceiling will reflect and therefore intensify the light from the flash. Conversely the light from the flash will fall off in an expansive hall, so the resulting exposure of a subject at the same distance and therefore with the same GN in each environment will render very different results.
NB This is a complex subject and this entry covers the basic principle without getting into complex maths and the inverse-square law.