What is a guide number (GN)?
Guide number is a measurement of how powerful a flash is. Here’s the scoop. A GN is usually measured in feet at ISO 100. It is computed by taking the best exposure from a series of test exposures and multiplying the distance to the subject by the aperture used. Since this is mostly subjective and done in a lab (and since GN is used by some people as the basis for flash purchases) most GNs are on the high side. You need to find your personal GN for the flash(es) you own.
