Pam, you could possibly pull this off with just the 580ex. I did this shot last Sunday in a Russian Orthodox church that had a modest amount of ambient light coming in from some side windows. I was standing on a short stool, using the 580 mounted on a Newton bracket. ISO 800, 1/30 sec, f6.3, shot RAW.
This is the full frame of the shot, no cropping, and no PP, although I did make global WB and exposure adjustments in ACR. The shadows really are not too bad, and I'll just have to do a little exposure layer adjustment for the album print. You can clearly see the flash fall off on the sides from the 580, but that portion will be cropped out, with any remaining dark background lightened in PS.
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