Guide Number

Figure 3.12 - The SB-800 shows thirds of a stop in fraction format.

Your flash has a power rating called a Guide Number that describes how powerful it is and how far it will shoot. As with all good things in photography, you can use a mathematical formula to figure out how your camera’s aperture should be set. Back in the good old days, we used to do manual flash calculations based on the power of the flash and the distance Aunt Sally was standing from it. Using this calculation, we could figure out how to set lens aperture with this simple equation:

To put this equation into practice, you would need to follow these steps:

1. Select your flash’s Guide Number

• SB-600 = 98

• SB-700 = 92

• SB-800 = 125

• SB-900 = 112

• SB-910 = 112