Ever tried to figure out how to get the perfect exposure? Old time photography pros probably already know all about the Guide Number, but Strobist has a quick guide to the “Guide Number”. It’s a handy bit of information, and I had a play with it this evening. You can dial in the f stop you want on the flash, the power level, and the iso, and it’ll tell you how far from the subject the flash should be. For example, on my SB-800, when set to ISO200, and 1/16th power, 50mm zoom, and f8, it says I should put the flash 2m away from the subject. Pretty nifty stuff. He even put up an image of an SB800 dialed in.