So, I've decided my next lens purchase should be somewhere in the 30-40mm range - I have the 50mm/1.4 but it is just too long for a general walk-around lens.
The problem is Pentax has not 1 or 2 but 4 lenses in this category. All of which are rated highly, and 2 of which have been included in a short list of the greatest lenses ever, comparing to the best of Zeiss and Leica.
So, here are the contenders:
31mm FA Limited 1.8
35mm FA 2.0
35mm DA Limited MACRO 2.8
40mm DA Limited 2.8 Pancake
If you had to have 1, what would it be? I could see a use for all of them, but I honestly think I'll only have room in my bag for 1 lens at this focal length.
These are tending towards being too tele to really be considered a normal on a digital body. Certainly most sane people would choose to be given the FA31. But see my thread on alternative Vivitar 28mm lenses.
close focus? -> DA35
size? -> DA40
value? -> FA35
if any of the above are not your considerations, i would choose the FA31. such a reliable lens, great for group photos, fast, build quality, bokeh, infinity focus sharpness, colour rendering...
The 31 is indeed a lovely lens to use! But if you're into manual lenses, I would highly recemmend you to find a tak, K or M-lens on the used market. I've been using a smc-m 50/1.7, and I must say that the 50 is even nicer to use than my 31, well, not for the results (the 31 is superior in IQ according to me), but for the feeling and that it is indeed a joy to use
As many people have said and will say, you will probably be pleased with any of the lenses you've listed