FA 31mm F1.8 Limited
With any lenses there are pros and cons. So my support for the FA31 is balanced on that score.
On an APS-C it is a true normal (35mm equivalent of a 46mm lens). This of course, sets it apart from the so called 50 something normals on APS-C, which are on this sensor really acting like a short telephoto. When on a full frame, it becomes a nice wide angle ..... oh it won't be long before we can embrace that format on digital
While a little soft wide open, with many marking the lens down on that score, the FA31gives options that no other Pentax lens can. I imagine this virtue is valuable for filming in poorly lit rooms. However, a stop or two down and it really starts showing its pedigree. Sharp is an understatement and can match any lens in its class. With auto focus, this lens starts to differentiate itself from its older manual alternatives (dare I draw comparison with the legendary SMC Pentax K 28/2).
Colours are reliable, punchy and accurate, but its rendering of out of focus scenes is what makes this lens really shine. The combination of sharpness and OoF rendering make two dimensional images burst into the third. A hallmark of a fine lens.
Not a small lens, but is quite compact for what it is (f1.8) and looks insignificant on a K01 (is it heresy to mate a legend with such a controversial camera body? Not in my experience). This lens camera combination is surprisingly discrete, an important factor for snaring candid shots.
Not many negatives to share - perhaps the colour fringing wide open on digital is an avenue for improvement. Not a big ask, and, in any case, easily repaired in PPing.
As for its rival, a fine lens I am sure, but limited by comparison for a 'normal lens'. No contest in my view.