The Guide Number (GN)
Technology - Technology
The Guide Number (GN) defines the amount of flash light which is necessary for a correct exposure of a a subject dependent on its distance from the flash and the chosen aperture:
GN = subject distance * f-stop
The max. GN of a flash unit is usually given for ISO 100 film. You can multiply/divide the GN by a factor of 1.4 per f-stop if use a faster/slower film. Based on this formula it is quite easy to calculate the max. subject distance for a correct exposure. e.g.:
max. GN of the flashlight: 40 (at ISO 100) 
ISO 400: film-factor = ~1.4 * ~1.4 =2 -> GN = 2 * 40 = 80 
chosen f-stop: 5.6
max. distance = 80 / 5.6 = ~14.2 meters

Note: If you have to go to the limit it is usually not such a good idea to place your subject at the calculated max. distance. Better take a bit off as a safety buffer (due to possible light loss effects + the actual GNs are always a bit lower than specified).

When it comes to choosing a new flashlight you have to be a bit careful. It is common practice (for marketing reasons) to provide the max. GN at the smallest coverage (=longest focal length) of a zoom-flash unit because such a focused flash light is more powerful (= bigger GN) than a wider setting at the same subject distance. Therefore the max. GNs of different flashlights are usually NOT directly comparable. The various flash feature pages on this site also provide the GN for 50mm coverage so compare the power of flashlights based on this value!

Note: The max. GNs are given either based on feet or meter so the numbers may differ dependent on the world where you live in (just multiply/divide the GN by ~3.3 in this case).

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