A number which serves as a guide to proper exposure when using flash. Also known as “Flash factor.” The number is based on a flash unit's light output and a digital camera's ISO sensitivity [..]

Indicates the power of a flash in relation to ISO film speed. Read articles that include this term

Guide numbers proclaim the light output of Electronic Flash units more realistically than Watt/seconds do, but still ignore such essentials as reflector design. The guide number divided by subject Distance represents the theoretical f-stop, but since you spent so much for that meter - exercise it.

The power output rating of a speedlight flash unit.

A number that relates the output of electronic flash when used with a particular speed film. The higher the guide number, the more the light output. Guide numbers, or GN serve as a way to calculate aperture when shooting flash in manual exposure mode. Dividing distance into guide number gives the aperture: For example, a flash with a guide number o [..]

A numeric expression of flash power. The higher the guide number, the greater the power of the flash and the greater the distance at which the flash can provide sufficient illumination for optimal exp [..]

The power of a flash in relation to ISO film speed. Guide numbers are quoted in either meters or feet. (To convert from meters to feet, multiply the metric number by 3.3). Guide numbers are used to calculate the f/stop for correct exposure as follows: f/stop=guide number/distance.  

A value used to calculate the exposure of a flash unit or light source. Also used as a criterion to describe the intensity of a flash or light source. A guide number of "1" is appropriate for ISO100 film at a distance of one meter.

Indicates the output, or power, of an electronic flash. You can use the guide number to work out the aperture or flash to subject distance by dividing either into the guide number.

The output power rating of a electronic flash unit. HAD CCD

A number used to represent the illuminating power of a photographic flash. When electronic flash is used, the camera shutter opens (i.e., the exposure begins), the flash fires, and the shutter closes; but because the flash duration is very short (and assuming relatively little ambient light), the 'shutter' does not control the amount of l [..]

Indicates the power of a flash in relation to ISO film speed. Read articles that include this term

A flash guide number indicates how powerful a flash is and how large the area it can illuminate.