Quote Originally Posted by peety3


the guide number of the 580EX II is 58m or 190', measured at ISO 100 and f/1. You'd need at least ISO 560 at f/5.6 if you're shooting subjects 190' away



f/5.6 is 5 stops slower than f/1(1.0, 1.4, 2.0, 2.8, 4.0, 5.6, etc) correct? Wouldn't you need an ISO of 3200 (100, 200, 400, 800, 1600, 3200) to maintain the guide number of 58m or 190'? I'm not being a wise-guy, I'm just trying to understand because I am not very well versed when it comes to flash-photography