A few weeks ago I picked up a new 85mm f/1.2L for my 5D Mark III (have used the body for over a year now) and started using it. Right away, I started to suspect there was something not quite right with it. Autofocus accuracy was incredibly spotty (still is). After trying and trying and trying to get consistently accurate results (mostly thinking it was just me), I finally took it back to the store to request an exchange for a different copy of the lens. The store does not accept refunds (fine, I didn't want a refund, just a working lens) and only accepts exchanges of defective product within 14 days. My first attempt to exchange was within this 14 days. They said they would have someone test the lens before accepting an exchange. He tested the lens, assured me it was a fine copy (and optically I believe it is, since I could live-view focus it fine), and said that its just a difficult lens to focus (no argument there). He even said it was sharper than their rental copy.
After doing a ton of testing, MFA adjustment, and more testing, this is what I've discovered: As focus distance increases, the autofocus error (front focus, specifically) increases. Wide open, the front-focus error increases faster than the gain in depth of field from the increased distance, so the farther out I get, the softer my pictures become.
So at this point I'm not sure what to do... I'm stuck with a rather expensive lens that's so off the mark most of the time that its not really worth it to me. Sure I can live view autofocus and it does great, but that's not the point. The person at the store who tested the lens suggested it may be my camera that is to blame. But I have 6 other lenses and never noticed this problem with several thousand clicks between them over the past year on this body. Granted, none of my other lenses are 1.2, but I do heavily use the 200 f/2.8L and the Canon 50 1.4. I did try the lens on my 5D Mark II for a bit, but the autofocus was so incredibly inconsistent on that body that I couldn't draw any conclusion as to what the trend was.
Any ideas how to proceed? I'm a bit frazzled, and tired of photographing test targets and playing with FoCal instead of being out and about making real photographs of real things.... I've never had such a disappointing lens purchase experience. And the 85 1.2L is so revered. I haven't been able to find anyone with a similar problem, and I can't seem to find many reports of bad copies of this lens at all for that matter....
Thanks for reading my long ramble.