Originally Posted by djdalibor
It will work as stated: it's an f/4-5.6 lens, and the 40D is an ISO 1600 camera (at best). The flash is a guide number 43m flash, so it can illuminate a scene of up to 43m away at ISO 100 and f/1. If my math and memory are correct, that's essentially equivalent to ISO 1600 and f/4, but assumes the flash is zoomed to the full telephoto position (105mm). Therefore, at the long end of the lens and the flash zoomed to 105mm (i.e. not covering the whole scene), you can light a subject 21m away, about once every four seconds. That's at max power and max aperture - you may want f/8 for better optical performance, cutting your max distance in half, and/or you may want lower ISO for less noise, again cutting your max distance in half.
I wouldn't want to be living on the edge like this. You'll need to keep a close eye on the flash confirmation light, and you'll spend a lot of time trying to choose the lowest ISO that gets the shot.
Reply With Quote