guide number


Also found in: Acronyms, Wikipedia.

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 ?
Speedlite 430EX flash unit replaces the Speedlite 420EX model with more features and higher performance with a maximum guide number of 43/141 (ISO 100, in meters/feet).
A more powerful flash with a maximum guide number of 58/191 at ISO 100 in meters/feet (at 105mm focal length), faster recycling, more consistent color and enhanced controls are among the improvements that all photographers, especially those who shoot digital, will find useful.
Smaller, lighter, faster and more powerful than its predecessor, the Speedlite 550EX flash, Canon has further enhanced the new model by reducing the full-power recycling time by about 25 percent, while increasing the maximum Guide Number power rating by about 50 percent.
Delivering great illumination power, the FL-50 has a guide number of 50 at tele settings and of 28 for wide-angle shots.
Speedlite 420EX has a maximum Guide Number of 42/138 (ISO 100, meters/feet) and the flash head's autozoom provides edge-to-edge coverage for focal lengths from 24mm to 105mm with bounce and swivel capabilities.
The built-in flash has a Guide Number of 38 (ISO100, feet; 11.