Today I dumped my new 50mm Sigmalux back to the camera store.
I tested a UX code 50L and it failed horribly in the danger zone at f/2.8 to f/4. I brought both 5D classic and 50D to the camera store (with 580EXII)
Oddly my 50D did not exibit as much back focus but it certainly consistantly had noticeable focus shift in the danger zone.
My 5D showed significant 1 inch back focus in the danger zone. This was very apparent and consistant with all of my test shots. I used the "human subject" using the eyes (lash) as a target. I find hand holding the camera with a human subject to be my quick "real life" AF focus test. With the head turned so that both eyes are not on the same focus plane I can quickly see if there is front/back focus issues. The eye furthest from the camera was in focus as I was in the 5+ feet danger zone distance at f/2.8 to f/4. At f/1.2 this copy was adequately sharp and pleasantly improved at f/1.4-1.8.
I walked out taking $1900 off the counter. Today was a very sad day for me. At least I helped my friend walk out with a T1i kit to start him on his new moneypit hobby 
