Originally posted by cpritchardv The FA 31mm, when put on a full frame, would look like putting a 21mm lens on my K-5II?
Yes, field of view-wise.
The 28-105 is reported to be very good, likely as good as the DA 16-85, a notch or so up from the 18-135.
The 24-70 is very good, I'm using it on my K-3II while waiting for the K-1 to arrive.
The FA 31mm is amazing, but it will be a single focal length. If you are OK with that then fine, but for me if you can only get one, get the 24-70 or the 28-105. Based on reviews (limited so far) that I've seen you will not be dissappointed in the 28-105 except that it is slow. If you need f/2.8 then get the 24-70.
Another thought is weight. The 24-70 is a beast. Compared to the 18-135 or 16-85 it is huge and heavy. Do you want to carry that around? If yes, go with the 24-70 if no go with the 28-105. I consider the FA 31mm to be a specialist lens, nice to have but not essential in my bag. YMMV.