In principle this should be a very nice lens. Correctly focused it produces sharp images with pleasant colours and contrast.
Unfortunately the one I got suffers from enough RSA to shift the focus noticeably backwards, even on distant subjects, between about 35mm and 60mm, for apertures f/5.6 to f/9. (Beyond this the DoF becomes big enough to absorb the shift, but diffraction starts to kill any gains.) This is in contrast to some reviewers who found the problem only exists at the 70mm end for close subjects. In mine the shift is enough to take the 'edge' off the images, and much better results are achieved manually focusing with Live View and holding the DoF preview button.
This is a very awkward way to focus.
For those of you wondering what this looks like, the triptych of 100% crops below was shot with this lens at 47mm, at a subject probably around 40m away (certainly not near, anyway). All were processed in DPP with the same recipe. The focus was on the window frame.
- A was focused manually and shot at f/4. Note the sharp definition around the window frame and flowerbed.
- B was taken with the focus dial untouched from A, now with aperture f/8. Note the softening of the window frame and the green foliage in the flowerbed before it. (A cheap kit-lens should be more than capable of this grade.)
- C was shot at f/8 by manually focusing in Live View, holding down the DoF Preview button. Subject is tack-sharp again.
I'm not aware of any fix for this. It can be worked around somewhat by focusing at 70mm and then zooming back out. Again --- Canon are you listening? --- a very awkward and tedious way to focus a premium autofocus lens. But as far as I'm aware this is a shortcoming of the design. Canon's own service has offered no solution. It could conceivably be fixed with a firmware update for the lens's microcontroller, by hooking into whatever routine that sets the diaphragm with instructions to also move the AF motor closer by a few steps.
The macro mode sort-of works if you're in a desperate situation, but it's very far from being a substitute for a real macro lens.
So in all a disappointing lens. My copy is capable of great results, but is marred by an optical flaw that hampers the usefulness of autofocus over a subset of its parameter space. On the plus side it is one of the few zooms I've bought that has shown acceptable optical alignment out of the box, and that's the only reason I'm holding on to it. But overall I can't really recommend it.