Originally posted by m42geo Doesn't it look at good at the DA* 50-135mm set at F4 at 135mm??
That's not a subject where it would be easy to tell, since there is not a lot in focus, and what is in focus is only toward the center, so e can't see how much better the Pentax would be toward the edges had they been in focus. And you didn't focus on the same spot, making exact comparisons of specific spots impossible. Nor is the scene exactly the same - the Sigma is actually at 170mm, not 135mm, giving it greater magnification and hence the illusion of greater resolution. Also, the Sigma is at f/4.5, not f/4, giving it more DOF (I doubt the Sigma is capable of f/4 at 135mm). And the 50-135 shot is noticeably overexposed - highlight detail is lost. That's not the fault of the lens; the flash system can't perfectly account for reflections, especially at that range.
But, since you asked: look at the little white balls on the ends of the bristles of the brush. Find the sharpest one on the Sigma, then on the Pentax. I'd say the Pentax wins pretty handily, if I am to believe any of those balls should have been in focus on the Sigma.
Is it the sort of difference that's going to jump out and grab you by the throat on every single shot when viewing at typical screen sizes? Of course not. And if you had been misled into thinking that would be had the idea that it would, well, no wonder you're disappointed. The Sigma *is* a pretty good lens already - if you don't need the f/2.8 or 50-70 range. And as others have noted, you're comparing the Sigma's best focal length against the Pentax's worst. It's only worth spending that kind of money on the Pentax if those slight improvements are worth it to you - or if you shoot f/2.8 a lot.