As mentioned by Ole you should read the in-dpeth review section. the FA31mm f1.8 is the highest ranked lens there. Read also the photozone.de review along the lines.
i have the FA31mm f1.8 and it is my preferred prime. The IQ is outstanding IMHO, and I love the lens for more reasons. It is my most useful prime: if I want an 'all-around prime', it will be the FA31mm. The focal lenght is spot on, not too wide, not to long. The large aperture makes also this lens a winner for low light.
Enjoy the FA31mm....