I've been wondering about this issue myself. Having shot a theater performance last week, I am becoming more and more convinced that I have a "bad' copy of a 24-105. I used two lenses- the 24-105 and the 70-200 2.8. In virtually every shot, the 70-200 has a measurable increase in sharpness. The 24-105 appears to be *slightly* out of focus, or soft, in most, but not all, cases. I would describe it as getting lucky with the focus at times. The 70-200, even wide open, is amazingly sharp across the range of focal lengths.
Because of my suspicions about this lens, I did a series of tests with it at a complete range of focal lengths and apertures with, get this, NO problems. I even did the whole series with the IS on and off. Granted, the tests were all done tripod mounted, not handheld, but if there was a general problem with this copy, would it not be revealed either way?
As a matter of note, I have also preformed the AF Microadjustment on this lens per the instructions on the
Northlight Images site. It was off a few clicks, but very minor.
What gives on this lens, and how would I go about having it professionally evaluated?