Flash Guide Numbers

The intensity of illumination of a flashgun's output diminishes as a subject gets further away from the source in accordance with the inverse square law. This states that when the flash-to-subject distance is doubled, the intensity of illumination of a subject is reduced to one-quarter. Consequently, more powerful flashguns are always an advantage because they allow a photographer to work at greater distances from a subject.

The effective power of a flashgun is quoted by manufacturers as a guide number. This is calculated under standardised conditions, and assumes that a speed of ISO 100 has been selected and that distances are quoted in metres. Guide numbers can be used to determine lens aperture for correct exposure at any given distance. For example, if a flashgun has a guide number of 32, and a subject is at a distance of four metres away, the correct aperture will be f/8.

Aperture = Guide number divided by flash-to-subject distance (using ISO 100 and a distance in metres).