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).