Went out this morning for a couples shoot. The 58 back focused on almost everything. Mine has always been bang on. I wonder if it being below freezing had any effect. Weird as hell.
Edit - I've never strobed with my 58 before; I wonder if that did it.
I was using the PW flex and mini, just the mini on the camera without a flash. Luckily it was just a series of pics and not the whole session. Oddly enough, the shots where it back focused by a mile, the flash/flex was no where remotely close. Literally about 150/200 yds away.
I have back focusing issues also when strobing with 85 1.4G and Sigma 35 1.4 Art. This only happens when I'm in Af-S mode. The second I put it on AF-C it gets much more accurate and almost no misses. I'm using YN622 with SB700.