A guide number (GN) indicates the strength (power and distance capability) of the flash in a flash bulb or electronic flash. GNs are usually given for ISO 100 and used mainly for the shoe mounted 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, and the intensity of this light falls off rapidly with distance from its source. This phenomenon is described by the inverse square law, which states that intensity is inversely proportional to the square of the flash-to-subject distance. For example:
You therefore have to take into account both the intensity of the flash and the distance from flash to subject. To calculate the aperture setting for the correct exposure, divide the guide number by the measured distance from the flash to subject (read from the distance scale on the camera’s focusing ring, once focused).
Aperture = Guide Number ÷ Distance to Subject (feet)
For example, if the GN of a flash unit at ISO 100 is 64 feet, for a subject at 4 feet the aperture would be f/16 (64/4=16); for a subject at 8 feet, the aperture would be f/8 (64/8=8); and so on.
Not all calculations are so straightforward. At 6 feet, for example: 64/6=10.66, but this doesn’t correspond to an exact aperture. You should thus choose the closest matching f-number (f/stop), in this case f/11. This might seem confusing, but makes sense when compared to our other results, as f/11 for 5 feet lies between f/16 for 4 feet and f8 for 6 feet.
Shutter speed used with flash 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 a focal-plane shutter camera, but any speed for an electronic flash with a leaf shutter camera; the faster the better if you plan on freezing motion). Some automatic cameras can automatically set the aperture according to the distance of the subject in focus. They may also prevent the shutter from firing if the subject is too close to or too far from the flash, which would result in overexposed or underexposed images.
It is worth noting that the actual range of a flash will change according to the environment and the ambient light. A small 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 could render different results in different environments.