Ditto on all of what Alpha says. And Misha, to answer your question, after reading all of the bad reports on Sigma zoom lenses, I feel very fortunate to have had my Sigma zoom on the camera rather than the plain-jane Pentax-A SMC 50mm f2 prime that came with the K1000. It seems from the reports that Sigma zoom lenses, especially when used with the aperture wide open, are optically pretty poor. From what I've read, the Sigma primes aren't as bad, which would explain why your 180 macro seems to perform well as opposed to your Sigma zoom. I've never found any explaination as to why the zoom lenses suffer and the primes don't. It would seem that poor optical perfomance would have to do with lens element quality, which should be the same for zooms and primes from the same manufacturer.