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Title: Poor auto focus compared to 24-105 f/4L IS
Review: I have tried two samples of this lens, and they both behave the same: severe back focus. Worse, when I try to compensate using the focus microadjustment feature of my camera (Canon 7D), the focus jumps around +/- several inches from shot to shot (measured at 70mm focal length and 6' subject distance). In practice, I sometimes got lucky and was rewarded with a very sharp image, but most of the time the images were soft to very soft. In contrast, the auto focus on the 24-105mm f/4L IS is extremely repeatable (I needed a +4 microadjustment on my 7D), resulting in sharp image after sharp image. Plus, the 24-105 is lighter, smaller, cheaper, and has an extra 35mm of reach. Yes, the f/4 is one stop slower, but it also has 4 stops of IS, which works very well. Low light motion situations are the only place where the extra f-stop of the f/2.8 would be an advantage, but your photos will still be soft.
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