guide number

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guide number

[′gīd ‚nəm·bər]
(graphic arts)
A number that relates the output of a light source (such as a flashbulb) to the sensitivity of a particular film; when the number is divided by the distance in feet to the subject, it gives the T stop at which the lens should be set.
References in periodicals archive ?
The center-weighted pattern provides larger guide numbers than other light distribution types at the same focal lengths.
Compact, lightweight and designed specifically for digital photography, the FL-36 achieves higher guide numbers than comparable SLR film cameras even while maintaining wide-angle light distribution.