Focus:
No fehlfokus, say most sits at 100%. Only in darker situations, the focus may be slightly wrong, but is only due when a 100% crop on. The autofocus is very reliable, especially fast. The speed is just under a USM lens. Only the volume is a little louder.
Image Quality:
At maximum aperture already good, just falls to the edge region from a little. With the 50mm 1.4 Canon can not keep up, but in my opinion.
Purchase recommendation