In a comment to yesterday's post Mike correctly points out that the ETTL II pre-flash should allow the camera to set the proper flash power regardless of whether the improvised bounce card was in place. I did a quick experiment where I took two pictures with the bounce card, one at 17mm and the other zoomed at 61mm. Pictured above are crops of the bear, resized to be the same size with the bounce card in place. Its pretty clear that the picture taken at 61mm is underexposed, for some reason the camera didn't set the flash power high enough. I tried changing the metering mode and it had no effect, so I'm puzzled. Is there some other factor at work? I didn't think the on-board flash of the Rebel XT had a zooming head but even if it did with the preflash the camera should be able to compute the proper flash power level. Maybe ETTL just sucks. Maybe this is one of those instances that all those folks who speak about the prowess of the Nikon flash system are referring to. I'm welcome to suggestions.
Pictured above is the uncropped versions, again the 17mm one is on the right.