I was looking at the two Photozone reviews for the Sigma and Pentax, and noticed that the two reviews had a couple of sample images that featured the same subject and composition. The lighting is different, but if you keep that in mind, I think they can still be useful for comparing the two lenses. Here are small versions of the images that you can click on to open the original, full size images:
After studying these images, and obscenely pixel-peeping them, I must say that I'm not seeing where the Pentax noticeably outshines the Sigma, except wide open at the extreme borders. Both sets of images look very good to me in terms of sharpness, contrast, bokeh, and color.
I think the gate image looks very good from both lenses. It's possible that the Pentax has slightly more pleasing bokeh in this instance, but it's kind of hard to make an exact comparison, since the leaves on Pentax image are bathed in sunlight, whereas on the Sigma image the leaves are in shadow. Also, the Sigma is at 1.4 while the Pentax is at 1.8, so it wouldn't really be an apples-to-apples bokeh comparison anyway. The big difference in this image is the leaves at the very bottom of the frame that happen to fall into the narrow field of focus. On the Pentax, those leaves along the edge are much sharper than the Sigma. The situation would be somewhat improved for the Sigma if it was also at f1.8, but the Pentax would still have the advantage here.
In the church image, the tree branches in the extreme upper-left corner probably look a bit better on the Pentax than the Sigma, but if you look at the church itself, it seems to me that the Sigma is noticeably sharper. This is most obvious in the bricks, and especially the details of the windows. Even the grass in front of the church looks like it has a little more detail in the Sigma image. And moving close to the edge of the frame, to the larger ball on top of the steeple, I don't see where the Pentax has a sharpness advantage.
I'm surprised that the Sigma would look sharper over so much of the frame, because according to the resolution charts, both lenses peak at f4, and then start to go downhill. But the Sigma church image is shot at f10, while the Pentax image is shot at f8, closer to the resolution sweet spot. But then again, the Photozone resolution charts
do show the Sigma achieving a higher maximum resolution, so perhaps this should not be so unexpected. I wonder how the comparison would have been if
both lenses were at f8.
But my point in all of this is that in the real world, the image quality differences between these two lenses are not nearly as pronounced as the lab tests might indicate. And it looks like what the Sigma gives up at the edges, it regains in the center.
For easy reference, here are links to the two Photozone.de reviews:
http://www.photozone.de/pentax/145-sigma-af-30mm-f14-ex-dc-pentax-k-review--test-report http://www.photozone.de/pentax/122-pentax-smc-fa-31mm-f18-al-limited-review--test-report