For example, doubling the guide number means the flash can illuminate an object at twice the distance, or for an object at the same distance can be used at one quarter the film speed. Doubling the guide number requires a quadrupling of the flash's power, as the area to be illuminated increases as the square of the distance. (cf. inverse-square law)
GN = distance × f-number
The guide number represents an exposure constant for a flash unit. For example, a guide number of 80 feet (24 m) at ISO 100 means that a target 20 feet (6 m) away can be fully illuminated with an aperture of (80 = 20 × 4) and a film speed of ISO 100. For the same guide number and an aperture of , the light source should be 10 feet (3 m) from the subject (80 = 10 × 8).
Guide numbers can be given in feet or metres, and are usually given for ISO 100 sensitivity.
Guide numbers do not depend on the focal length of the lens, but if the flash unit can be adjusted to match the focal length, this adjustment will influence the guide number: the wider the angle of the flash, the lower the guide number.
When comparing flash units, make sure to compare the guide numbers for the same ISO rating and the same focal length.
The flash-to-subject distance is longer when bounced flash is used, and the illumination is less when a diffuser is attached to the flash unit. In the former case it is possible to calculate the flash to subject distance that the light travels along the bounce path instead, but this still does not take into account the loss of light due to incomplete reflection. Most photographers apply a rule of thumb such as opening up by one to two stops.
f-number = GN / Distance
Some flash units have a dial on the back (in fact, a simple form of slide rule) or a chart to help in calculating the desired settings.
More recently, guide numbers are being used 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 luminous intensity (candlepower) or energy capacity (watt seconds). 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.