Flash exposures are interesting in that, with the exception of the lightning-like blasts of light produced by larger studio systems and the tinier “poofs” of light emitted from pop-up flashes, to the human eye all flash exposures are perceived as being equal in value even though they might differ by several stops. Unlike tungsten or daylight, even the most experienced, light-savvy eyes cannot accurately determine precise exposure times based on a momentary burst of light energy, and that’s where guide numbers (GN) enter the picture.
GN = Subject Distance from Flash Source x f/Stop
Guide numbers are based on a simple mathematical equation that states: the light output of an electronic flash is equal to the distance of the flash unit from the subject multiplied by the lens aperture, or f/stop. As a method of standardizing the process, manufacturers use ISO 100 and a flash-to-subject distance of 10' as fixed reference points when calibrating guide numbers.
An example of this formula: a flash unit with a GN of 40 would require an aperture of f/4 at a subject-to-flash distance of 10' (GN = 10' x f/4 = 40).
Note: Some less-than-scrupulous (and invariably third-party) manufacturers use ISO 200 as their base, which automatically increases the apparent power of the flash unit.
In the case of studio lights, manufacturers go one step further by using a standard reflector on the flash head as a third fixed parameter. If, when comparing flash power output levels between competing brands of flash units you note any variances from ISO 100, a 10' distance between your flash and your subject, or a reflector with a narrower focus angle than the flash system’s standard reflector, take note and make adjustments to your figures as necessary.
Unlike camera settings for ambient light, which entail setting combinations of aperture and shutter speeds, flash exposures are established by setting the lens aperture only, while leaving your shutter speed set at—or slightly slower than—the top sync speed. The exception to this rule is when you intentionally drag the shutter (i.e. use slower shutter speeds) in order to fill shadow areas with ambient light. If you want to use a larger (wider) lens aperture, you have to either dial down the power output or move the light further from your subject. If you want to stop down more you have to either increase the power output, switch to a more powerful power pack, move your lights closer to your subject, or resort to multiple “pops” of the flash system.
The good news, especially for those who break out in hives at the very thought of figuring out mathematical equations, is that digital cameras have greatly eliminated the need for doing the math. A quick test exposure instantly brings the image to life on your camera’s LCD, and by checking the histogram and over- and underexposure warnings, it’s easy to establish correct flash exposures.