I had 2 of these lenses.
One I purchased used with a small scratch on the front element. It always AF'd pretty near perfectly, no matter what body I used it on or what distance the subject. I used this lens a lot on my 5d2, 40d and other crop bodies. It was a great lens and I was happy with the results for years of use.
I also got a new one for a good deal, kept it as a backup. Never took it out of the box, the warranty expired years ago.
Went to use it recently, only to find that it's AF is useless. The thing misses focus really bad at various distances, tho sometimes it focuses just fine... Did a little experimenting and what I found was that it would nail focus at certain distances but miss (mostly backfocus) at various distances in between. It was repeatable enough that it seems to me to be a flaw within the lens and possibly how it communicates with the camera's AF system.
It's so bad that it backfocuses by about 2 feet on a simple high contrast target that's 10 feet away.
Seems Canon's quality control slipped on some versions of this lens. Too bad, it provided a decent IQ/$ ratio.
Not sure I want to spend $ on sending it in for a possible repair. Might consider doing my own take-apart and inspection.
Anyone have a similarly (mis)behaving 50/1.4 and any success with fixing it?