The guide number for an electronic flash measures its ability to illuminate the subject to be photographed at a specific film or sensor speed and
angle of view. 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, as the area to be illuminated increases as the square of the distance. ("cf." inverse-square law)
The guide number is the product of the maximum flash-to-subject distance and the
f-numberof the aperturethat 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). [cite web|title=Strobist page on Guide Numbers.|url=http://strobist.blogspot.com/2007/12/guide-number-your-free-flash-meter.html]
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 distance in the guide number calculation is the distance from the flash to the subject. The position of the camera is not relevant.
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.
Typical built-in flash devices on cameras may have a guide numbers of 15 feet (5 m) or less, while high-powered flashes can have guide numbers of over 250 ft or 75 m. [cite web|title=a Metz potato-masher flash with a GN of 54, @ 50mm, or 76 @ 105mm zoom, ISO 100.|url=http://www.metz.de/en/foto-elektronik/metz-mecablitz-modelle/sca-blitzgeraete/mecablitz-76-mz-5-digital/datenblatt.html] [cite web|title=a Canon flash with a GN of 58 @ 105mm, ISO 100.|url=http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=ModelTechSpecsTabAct&fcategoryid=141&modelid=14998] [cite web|title=a Nikon flash with a GN of 38 @ 35mm, and 56 @ 105mm, ISO 100.|url=http://nikonusa.com/Find-Your-Nikon/Product/Flashes/4801/SB-800-AF-Speedlight-.html#Tech%20Specs] [cite web|title=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.|url=http://sunpak.jp/english/products/pf20xd/index.html] [cite web|title=a Nikon flash for smaller cameras with a GN of 30m, "18mm zoom head position", ISO 200.|url=http://nikonusa.com/Find-Your-Nikon/Product/Flashes/4806/SB-400-Speedlight-Unit.html#Tech%20Specs] [cite web|title=Canon flash with a GN of 190 feet (58m) at 105mm zoom, ISO 100.|url=http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&fcategoryid=141&modelid=10514#ModelTechSpecsAct] [cite web|title=Mecablitz 76 MZ-5 digital with guide number 249ft (76 m) at ISO 100 and 105 mm.|url=http://www.metz.de/en/photo-electronics/mecablitz-models/sca-flash-units/mecablitz-76-mz-5-digital.html]
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 viewof the lens, making it difficult to compare different flash units based on the guide number alone.
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