i know that some will roll their eyes when they see another post regarding backfocus on this lens. however: it is a very real problem.
i bought this lens for use on 5d. immeditaely experienced difficulty focusing at various apertures (not just 1.2) and various distances (not just closest focusing distance).
a canon tech rep in L.A. looked at the lens and found that, while it seemed to be fine on a mark III body, he could replicate my problem on a 5D. he suggested taking to canon for calibration.
i will keep my canon service center story short but will say that i was very, very unhappy with how my problem was handled. lots of driving back and forth and, finally, the lens returned to me in the same condition as when i dropped it off.
finally, someone head my complaint and i brought the lens back to canon 1 final time.
HERE IS WHERE IT GETS INTERESTING:
1. the technician DID find that it backfocused.
2. he said that 4 CM was "within the spec" for the product.
i found this preposterous and asked him to have a crack at calibrating it.
in 20 minutes he was able to get the lens focusing properly on my 5D.
this 20 minutes was the culmination of 2 weeks of persistance.
- my copy of the lens did backfocus (i actually tried TWO copies: both with this problem)
- it was correctable.
- it was a terrible ordeal
- i have yet to determine if it was worth the time and aggravation.
this lens may yet go back to the store.
i am not a pixel-peeper and i rarely put my lenses through precise testing. this lens had GLARING issues, to my eye. perhaps my "review" will spare someone else a bit of grief.