Originally posted by savoche As for sharpness I believe the Sigma 8-16 is sharper than the 10-20. Just to complicate things further
To complicate things even further there's two versions of the Sigma 10-20, one which is faster, and there's a Tamron 10-24 with a wider range. And don't forget the Pentax DA ~12-28 scheduled to come out in the next year or so, as well as a Samyang 10mm f2.8.
While the Sigma 8-16 has a reputation for being as sharp, if not sharper, than the other zooms, I'm skeptical whether anyone would notice the difference on screen or in prints at normal viewing sizes. All these lenses are plenty sharp enough. So there's no compelling reason to pick one or the other because it's "sharper." Pick the lens that fits one needs, shooting style, aesthetic goals, etc. rather than which one is the "sharpest." For example, if you already have 16 and above covered and you don't mind have a lens without filter threads, the 8-16 is a great option. But if you need the 16 to 24 area covered or want to use threaded filters, it's not a great option, irregardless of how sharp it may be.
I suspect in terms of sheer visible IQ (i.e., what you see on screen or in prints) the DA 15 is the best of the bunch. But the differences are slight and I don't think you could go wrong with any of the options.