If you have ever shopped for a flash or read a review, you may have noticed a section of the spec called the guide number or GN. While this value is a measurement of power that the flash has and allows you to compare flash models, is this number useful to you at all? In this article we dig into the guide number and see how we can use this number help us dial in our flash and camera settings using some math instead of just guessing and retrying until you get what you want.