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Guide number
The guide number for an electronic camera flash measures its ability to illuminate the subject. A higher guide number indicates a more powerful flash - doubling the guide number quadruples the power (see inverse-square law), which means the flash can project twice as far, or can be used at one quarter the film speed. Inbuilt flash devices may have a guide numbers of 15 or less, while some high-powered flashes have guide numbers greater than 100.
The guide number for an electronic camera flash measures its ability to illuminate the subject. A higher guide number indicates a more powerful flash - doubling the guide number quadruples the power (see inverse-square law), which means the flash can project twice as far, or can be used at one quarter the film speed. Inbuilt flash devices may have a guide numbers of 15 or less, while some high-powered flashes have guide numbers greater than 100.
Guide created: 12/03/07 (updated 05/31/08)
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