I just got a new Canon 70-200mm f4 L for my Rebel XT. I noticed that my bird pictures are not very sharp. I also have seen numerous reports of front or back focusing issues some people have had with this lens. I did a test tonight. I used a tripod and mirror lockup. I fosused on a white box with smallish black text from about 8 feet away at 200mm and f/4.0. I took one shot in autofocus and one shot manually focused. The manual focused shot was tack sharp and the auto one was slightly off. I repeated this several times with the same results. I then went back, and in autofocus mode, I unfocused CCW and shot, then unfocused CW and shot. I did this several times. This time, even tho I was in autofocus for every shot, a few of the shots were very sharp. The rest looked slightly off like before. I am very certain there was no camera shake or anything like that involved, and the fuzziness did not look like camera shake. Is this a bad lens or am I obsessing too much? If it is bad, should I just send it back, or go thru the hassle of getting the lens/camera recalibrated? I'd buy one from a local store, but we only have Ritz which is overpriced IMO.