The guide number is given in feet (or meters) at a given ISO, usually ISO 100. The higher the guide number, the more powerful the flash. You can use the guide number, along with a little math, to determine the correct aperture to use.
|Title||:||Exposure and Lighting for Digital Photographers Only|
|Author||:||Michael Meadhra, Charlotte K. Lowrie|
|Publisher||:||John Wiley & Sons - 2011-06-09|