« on: July 21, 2013, 04:55:35 PM »
I had bought a used 24L II and it was the focus was off. It didn't come close to focusing accurately until f/5.6. Live View was a lot more accurate. Sent to Canon, and the problem was fixed. Used it on a 20D and it was fine using the center point. Used it on a 5DII, and the center point was fine. Use it on a 5D III and it is fine at a lot more focus points.
In general, the target should have high contrast. In low light, that is tougher and the focus accuracy falls. Focus beam helps.
If your not getting good center sharpness in good light (go outside on a bright day, ISO 100 with shutter speeds in the 1/1000s) at your AFMA CALIBRATED distance, then it would make sense to have it checked out.