FA50 f1.7 really shines in low light, but daylight is another matter. On the Q, the purple fringing (which isn't noticeable on my Kx or K01) is blatant, ugly, and a great many pixels wide. The purple fringing doesn't go away even when the lens is stopped down to f8. It's still amazingly sharp, but if there's any kind of high contrast purple fringing comes in to play. The best way I've found to deal with it is to shoot at f4 and set exposure compensation to -1 stop. Then I can bring the brightness back up in post. Not ideal in the least.
But for dimly lit stages, concerts, or anything in low light, it does very, very well.
Edit - after playing with the lens a bit more, my latest batch of photos have no fringing at f5.6. I'm not sure what was going on before, but today I got some really nice photos, so I'm going to play with this lens a bit more before giving up on it in daylight.
However, at f1.7 in daylight, I have noticed that the entire image has a purple color cast and is very soft. It's not soft at f1.7 in darkness, just in daylight, where the purple fringing seems to effect the entire image.
For example:
LOTS of fringing at f1.7, and a very purple color cast.
Charles.