I have problem with this lens (50mm f1.4). I sent it to canon 7 times and couple times with my body. I still have the same problem with this lens. I also compare this lens with L series (24-105, 17-40, 70-200 f2.8 IS)and non L series (28-135). My 28-135 IS will give me sharper image than 50 mm with the same set up.
If I use f11, 1/1000 and flash, I will get pretty sharp picture. I will get soft/back focus for f2.8 and bigger opening with speed 1/80 or slower. I do not recommend to buy this lens if you want to use for low light with no flash. I wish I get a good copy of my lens.