Guide Number

GlossaryGlossary of Photography TermsGuide Number

The guide number represents an exposure constant for a flash unit. Generally, the guide number indicates how powerful the flash is, and hence the size on an area it can illuminate. The higher the guide number, the more powerful the flash.

The formula for calculation:

Guide Number = aperture x distance of the object from the flash.

External flashes have guide numbers of 20 to 50; built-in flashes have guide numbers between 10 and 20.

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