Originally posted by tcwilson Thank you again for all your insights! From a flexibility point of view, I am fairly well covered at at the wider end of focal lengths, although not with fast glass. I have the Sigma 10-20mm 4-5.6, Sigma 17-70 contemporary (It is on the camera most of the time. It is a very nice lens. Perhaps the other question is how much better is the 18-35 than the 17-70. I suspect the point of diminishing returns is fast approaching in that arena.) I also have the Pentax 18-135 (Which I bought especially for when conditions were going to be iffy), and the FA-50 1.4 (Which I love). Yes, its a legitimate question about do I really need another lens. But I suspect people here will understand. :-) So I am really hunting for fast glass and amazing IQ. But from the comments, it sounds like both the 31mm and the Sigma Art offer that, although with slightly different strengths and weakness. So it seems to come down to the benefits of the much smaller Pentax and the fact its a FF, compared to the benefits of a zoom provided by the Sigma. In the scheme of things, its a nice problem to have. In case anyone wonders, I am shooting with a K-3.
Okay, I own the Sigma 17-70 Contemporary lens too and it is outclassed by both the 31mm and the 18-30mm. They are not even playing the same game. I can't even remember the last time I took my 17-70mm Sigma out to shoot. I typically take my 18-35mm 1.8, 50mm 1.7, 77mm 1.8 L, and 70-200mm 2.8 OS Sigma. If I and other lenses the next ones in the bag are 8-16mm Sigma, 31mm 1.8 limited, and 100mm Macro 2.8.
The 17-70 is one of the best kit lenses / walk around lens options available, but I prefer the lenses above due to the image quality and consistent fast aperture delivered. I stopped using my 17-70 Contemporary the day I purchase the 18-35mm and I haven't looked back. BTW, I haven't had any focusing issues with my 18-35mm but many owners apparently have.