Originally posted by mikesbike Well, the Sigma 35mm 1.4 "Art" lens is certainly a fine one, perhaps the best lens overall Sigma has made. I will stick with my modest FA 35mm Pentax for fine IQ since I don't often need faster than f/2.
But the original question is a FF replacement for the very useful and nice DA 21mm Ltd, so consideration has to address similar usage, including focal length. Any 35mm lens is not quite there, because of a substantial difference in focal length and field of view. The FA 31mm Ltd comes closest, and presents very fine quality IQ. Every test review has given it a very high rating. Photozone gave it top honors for sharpness and bokeh, although their test was only for APS-C use. This lens is not as fast as the Sigma, but much faster than the DA 21mm Ltd. It is also much smaller and lighter than the Sigma, yet with a noticeably wider field of view, which, on a FF body would be near that of the DA 21mm Ltd on an APS-C body.
Otherwise, a used FA 28mm f/2.8 in top condition might be a good FF alternative. It would definitely offer more WA, and would be close in aperture- f/2.8 compared to f/3.2 and has also received very high reviews. I have one and like it very much. It does not have the build quality of a Limited, though.
Although I much agree with these views, let us bear in mind the fame of the FA31 originated in the times when none of Pentax-brand owners ever planned making an FF digital SLR. FA31 is certainly excellent with APS-C but I believe it is not the top lens for high resolution FF sensor; it is enough to study the MTF figures. The FA31 was tested by Photozone at 10 Mpx and the center resolution at f/1.8 was 1848.5 lw/ph. Sigma A 35 as tested on an FF Nikon gave center resolution of 3262 lw/ph at f/1.4. You don't need pixel-peeking to determine that: It is clearly visible in pictures taken.
Although certainly the FA31 is a respectable lens, its high price and the old design make the lens natural to be reused by existing owners on the K-1 body but I would think thrice before I recommended the FA31 to a new user. Why to pay twice what you spend for Sigma to buy a lens that is outresolved by the K-1 sensor? And well, a 6-degree angle of view difference between both lenses? Is it worth paying the double price? I doubt and that's why I chose the Sigma for my coming K-1 body though I could afford the Pentax Limited lens.