Buying a new flashgun

When buying a new flashgun, it is important to look at its guide number (GN). This number indicates the power of the flash, but many photographers don’t understand how the number is calculated and what it represents.

The GN is the maximum distance that a subject can be located from the flash and still be illuminated, at an aperture of f/1 and (unless otherwise specified) at an ISO sensitivity setting of ISO 100.

Given that the maximum power output of the flash is a constant, its GN can be used to calculate the aperture needed to correctly expose a subject at any given distance.

Aperture = GN/distance from subject. So, for example, take the Canon Speedlite 580EX II, which has a GN of 58m at ISO 100. Using the equation aperture = GN/distance, a subject that is 14.5m away from a Canon Speedlite 580EX II will be correctly exposed at an aperture of f/4 at ISO 100.

Until recently, most flashguns had an f-number calculator on the back. This allows you to select the ISO setting and distance from the subject, and it would then tell you the correct aperture.

While using an exposure calculator may seem somewhat redundant in an age of sophisticated TTL flash systems, knowing the exact relationship between the flash output and distance can be very useful.

See pages 44-45 of AP dated 8 August 2009 for an example of this technique in action

by Richard Sibley