Hi,
I use Canon DSLRs (7D and 550D) with a standard 3 zoom approach. I'm very happy with my wide angle (the 10-22mm) and telephoto (the 70-300 IS USM) - both are sharp, with well-controlled geometric distortions and low CA distortion. However, I've never really found a mid-range zoom I'm happy with.
The 17-85 mm IS is OK, and I've done a lot of decent work with it, but it's quite soft at the corners and the Chromatic Aberration is quite bad, especially at the wider end.
I also have the 15-85 mm IS, but I've been a bit disappointed with it. The geometric distortions are quite bad, arguably worse than the 17-85, and while CA is better than the 17-85 it still needs a fair bit of correction. However the worst problem is that images seem to be very soft towards the edges. I've even returned it to Canon for recalibration but it's still not brilliant. Obviously this could just be my example, but I have no way of telling...
Is there a better option? I want an autofocus lens with IS covering at least the 17-40 range, with a similar optical quality to the 70-300mm IS USM. The Canon 17-40L and 16-35 are no good because they don't have IS. I tried the 17-55 f/2.8, but I found it very heavy and not that much sharper than my existing lenses. I've also tried the 24-105L, which I quite like, but on my cropped-sensor cameras it's frustrating that it's only 24mm at the wide end.
Are there any other options? Please note that I'm not interested in answers which boil down to either "use primes" or "change camera brand", but I might be prepared to look at other lens suppliers.
Thanks
Andrew