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.
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)
Read more about Guide Number: Value, Applications
Famous quotes containing the words guide and/or number:
“Whatever we’re doing, whoever we are, it isn’t enough. . . . Little wonder we have trouble finding role models to guide us through these shoals. No one less than God Herself could be all the things we’d like to be to all the people we’d like to feel approval from.”
—Melinda M. Marshall (20th century)
“Among a hundred windows shining
dully in the vast side
of greater-than-palace number such-and-such
one burns
these several years, each night
as if the room within were aflame.”
—Denise Levertov (b. 1923)