I have owned/tested several copies of this lens from 2006-2008, the most recent one having a date code of UW07xx and have found that all have the infamous focus shifting issue.
Focus Shift Issue: When shooting from within 4-5ft from the subject and closer all the way to MFD, at an angle (say 45 degrees), and between the apertures of f2-f4, even f5.6, as you stop the lens down, the focus plane shifts towards the back. For the lack of a better description the lens progressively back-focus as you stop the lens down from f1.2 to f5.6 so what was in focus at f1.2 is OOF by the time the lens is stopped down to f2-f5.6. At f8 the shift is hidden due to widening DoF. What this means is that focusing on the left eye will result in an OOF left eye and the left ear being in focus. Completely unacceptable.
Again, I stress the point that this is situation specific. When you are further away from the subject, the problem is still there, but is "hidden" by DoF limits.
But the problem is very real. Canon have known at least since May of 2007, promised to "take a look into it". Over a year later, nothing.
Good luck sending in the lens to Canon for calibration. They don't have the medicine for this illness! Granted my copy came back a little better than before (used to backfocus very badly and not even the Micro Adjust feature of my 1DMKIII could compensate for it). I sent mine in 3 times and grew tired of this exercise.
On the seldom occasion that the lens nails the AF, it can be as sweet as the 85L, which makes it even more frustration. So much potential...
I would recommend purchasing this lens from a place you can easily return it. If you are lucky enough to find a copy that's acceptable (again, the shift problem is inherent in the design of the lens, so you cannot completely avoid it) then hang on to it for dear life.
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