What a nightmare.
I had heard about the backfocusing issues on this lense so when it arrived i tested it quickly in the garden, seamed ok.
Then did two weddings with it and had issues with 40% of pictures taken with it. an object just behind the subject was in perfect focus and the subject was soft focus, time and time again.
The 1st wedding was a 12hr day so did not have time to check on the PC, looked ok on camera.
Canon solution to their poor testing and generous tolerance on the focus circuit is to allow cameras from the 1dMKiii series on to recalibratre upto 20 specific type lenses.
They are def having a laugh.
Buy at your own peril