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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 standardized conditions, and assumes that a speed of ISO 100 has been selected and that distances are quoted in metres (or feet). The calculation remains the same whether the subject distance is quoted in metres or feet provided use of units is consistent - ie it is remembered whether the guide number represents metres or feet! 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 (metres), and a subject is at a distance of four metres away, the correct aperture will be f/8.The focal length of the lens has no bearing upon the guide number because the field of view does not affect the intensity of illumination at a given subject distance.

Guide Number (GN) Aperture (f/stop) Subject Distance Calculation
 80 feet  f/8 10 feet GN = 10 x f/8 = 80
 120 feet  f/8 15 feet GN = 15 x f/8 = 120
 160 feet  f/8 20 feet GN = 20 x f/8 = 160

The table above shows calculations in feet for three different flashguns having guide numbers of 80, 120 and 160 (feet). With a common aperture of f/8, it can be seen that the higher guide numbers (more powerful flashguns) allow subjects at a greater distance to be photographed at the same aperture.

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


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