Guide number

The guide number for an electronic flash measures its ability to illuminate the subject to be photographed at a specific film or sensor sensitivity and angle of view. A higher guide number indicates a more powerful flash. The guide number (or "GN") can be used to compute flash power, subject distance, or camera lens aperture for correct exposure.

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 sensitivity. 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).

Guide number is frequently abbreviated as "GN".

The guide number is the product of the maximum flash-to-subject distance and the f-number of the aperture that will correctly expose film or a digital sensor with the specified sensitivity.

This simple relationship is well-defined because the brightness of a flash falls off with the square of the distance, but the amount of light admitted through an aperture decreases with the square of the f-number.

The guide number represents an exposure constant for a flash unit. For example, a guide number of 80 at ISO 100 means that a target 20 meters away will be correctly illuminated with an aperture of f/4 (80 = 20 × 4) using a sensitivity of ISO 100. For the same guide number and an aperture of f/8, the light source should be 10 meters from the subject (80 = 10 × 8).

Guide numbers do not depend on the focal length of the lens: the distance a flash can illuminate does not depend on the angle of view of the lens. However, some flashes have the capability to "zoom with the camera" and concentrate their light into a narrow beam for use with a telephoto lens. Since the light from the flash is more concentrated, this increases the guide number. Manufacturers typically advertise the guide number for their flashes at the narrowest setting. For instance, the Olympus FL-50 has a guide number of 50 when set to its narrowest setting, but significantly less when illuminating a wider area.

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