Guide number
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The guide number for an electronic flash measures its ability to illuminate the subject to be photographed. A higher guide number indicates a more powerful flash. 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 - see the inverse-square law).
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[edit] Value
The guide number is the product of the maximum flash-to-subject distance and the f-number of the aperture that will correctly expose a specified speed of film.
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 f/4 (80 = 20 × 4) and a film speed of ISO 100. For the same guide number and an aperture of f/8, the light source should be 10 feet (3 m) from the subject (80 = 10 × 8). [1]
Guide numbers can be given in feet or metres, and are usually given for ISO 100 film.
Guide numbers are tied to the focal-length of the lens, however: A flash that can illuminate a wide-angle scene is throwing its light wide. A flash that instead throws a tight beam, will illuminate a smaller area, and do so sufficiently at a larger distance.
Always check what focal length the guide number is for, as well as the ISO, to be able to compare between units/brands.
[edit] Distance
Because the distance is measured from the flash unit to the subject (instead of camera-to-subject) the guide number calculation will be inaccurate for camera placement if the flash unit is mounted off-camera through the use of a cord or a wireless technology. However, the subject will be illuminated correctly if the flash is placed at the calculated distance, and the camera is set at the calculated f-number. Camera-to-subject distance will not be relevant for purposes of exposure.
The flash-to-subject distance will also be inaccurate when bounced flash is used, or when a diffuser is attached to the flash unit. The former can be corrected by considering the flash to subject distance that the light travels along the bounce path instead. For the latter case most photographers apply a rule of thumb such as opening up by one to two stops.
[edit] Examples
Typical built-in flash devices on cameras may have a guide numbers of 15 feet (5 m) or less, while some high-powered flashes have guide numbers greater than 130 feet (40 m). [2] [3] [4] [5] [6]
[edit] Applications
Guide numbers are especially useful in dealing with manual flash units when calculating the aperture for a particular exposure or the maximum flash range for a given aperture. In practice, it's normal to know the guide number and the distance and need the aperture, so the equation is rearranged to:
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.
[edit] References
- ^ Strobist page on Guide Numbers..
- ^ a Metz potato-masher flash with a GN of 54, @ 50mm, or 76 @ 105mm zoom, ISO 100..
- ^ a Canon flash with a GN of 58 @ 105mm, ISO 100..
- ^ a Nikon flash with a GN of 38 @ 35mm, and 56 @ 105mm, ISO 100..
- ^ a Sunpak flash for pocket cameras, with a GN of 20, but no information about what angle of coverage that is measured at, ISO 100..
- ^ a Nikon flash for smaller cameras with a GN of 30m, "18mm zoom head position", ISO 200..
