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My guess, however, is that you would not be able to distinguish between photos taken with the new Sports model (fully micro adjusted) from those shot on an earlier version of the lens. There are plenty of other good reasons to justify an upgrade to the S model (better weather sealing, USB software updates, better tripod ring, nicer looks to name a few).I just don't think IQ is one of those reasons. I asked this very question of a Sigma rep at the CES show in Las Vegas earlier this year and he acknowledged as much.
I just want to make sure we're both talking "oranges" here...are you (and the rep) saying their is no IQ difference between the two OS versions of the 120-300mm, or are you (and the rep) saying there are no IQ differences between the non-OS and OS versions? I'm suspecting that you and the rep might have been saying different things.
As I stated earlier in this thread, there is a distinct improvement to both IQ and AF between my EX DG non-OS, and my non-Sport OS, the OS version being, um, "better". YMMV.
To be clear, the two original non-OS 120-300mm lenses, the EX and EX DG, were both optically identical except for the DG lens coatings. The two OS versions, the non-Sport and Sport, differ as Lars points out, though, optically the two OS versions are optically identical. The non-OS and OS versions are not optically identical.
Just trying to keep confusion at bay here!