Hi Richard:
I can’t directly address your question, as we don’t use the 580 EX, but we do own three 550 EX’s and we use them extensively with the 500 mm f/4 and the Better Beamer combo and with other long lenses. I’ve been very happy with them in this context for our bird photography and always use them as fill-flash in the usual range of -1 1/3 to – 2 stops. For some example images, take a look at our web site in the North American Birds gallery…….all of these were shot with the 550 as fill-flash and most were with the 500 mm f/4 (with and without a TC).
On the other hand, I have heard from other guys that I often shoot with that the 580 EX works better with the newer 1 series bodies than the 550 as Canon apparently changed the flash algorithms. (By “better” I think they mean more consistent results in general photography, not specifically birds.) I can not comment either technically or practically on this as we don't have the 580, but if you are using a 1 series body you might want to check into it some more.
Hope this helps.
