Originally posted by reeftool The Sigma has a lot of good reviews and is also a macro along with being a hundred dollars less than the Pentax. I'm really not sure if the capability to fine adjust the lens to the camera will make that much difference.
The Sigma is not a true macro lens, it's only 1:2.3. The Pentax does have close focus ability, but not as much at 1:3.2. Neither lens is exemplary for CA, vignetting and especially distortion, so the K-7 corrections should make quite a difference.
Originally posted by reeftool As I compare the Pentax and Sigma 17-70 lenses, the Sigma is faster on the short end (f2.8) and has macro which makes it a little more versatile as a walk around.
You seem to only be giving the plus side of the Sigma 17-70. It's also not as sharp as the Pentax and not as good at colour rendition and contrast. The Pentax is silent when focussing, which could be a major advantage in some situations, although reliabililty of SDM has been questioned lately.
The Sigma is a very good lens, but nothing I've seen would make me choose it over the Pentax, even given the difference in price.