Pentax SMC-F 50mm f1.7 on the Q:
In low light, this is a wonderful lens. It can be used wide open with more than satisfactory results.
HOWEVER, in daylight, using it wide open will result in a soft image with a LOT of purple fringing. In fact, the entire image will have a purple color cast. It delivers sharpest results at f5.6, but falls apart at f8. OOF specular highlights look very, very odd at f1.7 (You can't really tell from my image below because they're not that OOF, but the specular highlights look like snowflakes)
Low light wide open:
Bright light, 50mm f1.7:
100% crop, 50mm f1.7:
Bright light, 50mm f5.6:
100% crop, 50mm f5.6:
Note that the low light photo was edited - some NR, sharpening, color balance, and saturation.
The images of the telephone pole are straight out of the camera, JPEG, no edits except for resizing and cropping.
Charles.