That's a buncha alphabet soup, eh? I just happen to be upgrading from the Sigma to the Pentax, and I thought this might be a good time to do a quick head-to-head test. First things first, though, the significant differences in the lenses. The Pentax is weather resistant, the Sigma is not. The Pentax is SDM, the Sigma is screwdrive (noisier). The Pentax goes two millimeters wider - which wouldn't seem like all that much, but is significant in image framing. The Pentax is enormous by comparison, and heavy. The Sigma focuses closer than the Pentax - although the Pentax focuses closer than I expected, down to about four to six inches.
But what's important to me beyond those things I listed is image quality. Now, I'm not going to stack up a pile of side-by-side images. This is not an exhaustive shootout; just a quick head-to-head between a lens and the one that is replacing it. I'm going to tell you that from my position as occasional measurebator and general curmudgeon, in most applications, the real-life difference is one of personal choice.
With manual flash settings and the same aperture and shutter speed, the Sigma produced consistently darker images, by almost half a stop, all the way from f2.8 to f11. If you're shooting raw, this is well within adjustment range, and I promise if I put the pix up here side by side, some percentage would say the Pentax was overexposing, not the Sigma underexposing. Fair enough.
I am, however, going to provide a couple of images for side-by-side viewing. First the full frame images:
Pretty similar in appearance. The Sigma is a little warmer, maybe the Pentax a little more contrasty. When you go pixel-peeper on it, however, the difference really shows up. The Pentax is crisp and contrasty, but the Sigma goes all soft and dreamy - 
Of course that doesn't mean the Sigma is a bad lens. After all, the Pentax lens costs almost twice as much. As I said, through most of the range of test images I shot, the difference was one of taste, no obvious differences in IQ. The Pentax focuses almost completely silently, and much faster and more accurately, but that's to be expected with the SDM motor.
Many people have said - mostly in 2007 - that the Pentax 16-50 has quality control issues. They may have resolved them, or I may have simply gotten a "good one", but I can't find a single flaw.
All in all, I think the Sigma is a good value, a great bang-for-the-buck purchase, but, at least in the instances of the two lenses I received, the Pentax is clearly superior. I'll be selling the Sigma soon. Watch ebay if you're interested!
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