Pentax FA 35mm f/2 vs FA 31mm f/1.8? – "A Mighty Mouse", who has owned both lenses, says:
Those who own and have only owned the FA35 will sing its praises and tell you the FA31 is too expensive. Those who have owned both will tell it like it is – the FA35 is a good lens for the money (or was – it is now out of production). The FA31 is a better lens… faster, sharper, better bokeh, quicker and quieter AF, much better build.