Guide number
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The guide number for an electronic camera flash measures its ability to illuminate the subject.
[edit] Formula
The guide number is the product of the maximum flash-to-subject distance and focal ratio that will correctly expose a specified speed of film. The guide number represents an exposure constant and is the product of distance times f-number. For example, GN 80ft @ ISO 100 means that a target 20 feet away can be fully illuminated with an aperture of f/4 (or 20 x 4) and a film speed of ISO 100. For the same guide number, 80, and an f-number of 8, the light source should be 10 feet (80/8) from the subject.
Guide numbers can be given in feet or meters, and are usually given for ISO 100 film. As the film speed doubles, the guide number increases by a factor of 1.4; as the film speed is halved, the guide number is reduced by a factor of 0.7.
Because the distance is measured from the flash unit to the subject (instead of camera-to-subject) the guide number will be inaccurate if the flash unit is mounted off-camera through the use of a cord or a wireless technology. It will also be inaccurate when bounced flash is used, or when a diffuser is attached to the flash unit.
[edit] Applications
Guide numbers are especially useful when dealing with manual flash units to calculate the aperture for a particular exposure or the maximum flash range for a given aperture.
More recently, guide numbers are used more to market the power of a flash unit. However, guide numbers are not measurements of the amount of light output of a flash device, which is normally measured in terms of candlepower or energy capacity. Some modern flash units come with zoom attachments that can diffuse or concentrate the light output according to the angle of view of the lens, making it difficult to compare different flash units based on the guide number alone.
