I bought a 50 f1.4 USM here for like $150. It would stop down but would not AF at all (see video here)
Oh here is what I learned. the 50 1.4 is not a ring type USM, it is a Micro USM, Also the front of the lens moves In and out when you focus. A word of advice. Buy a HOOD also Park the lens in Infinity. IF you dont the front of the lens will protrude the barrel. This is a problem because of the design of the lens. Any force on the front of the lens can bend the gear assembly causing the USM to fail. It causes a pinch point at the gears and and the USM will soon fail. Here is the OLD bound up one with a USM I took out of the lens I bought here.
I am happy to say the Lens is not AFing like NEW. However I have more in parts and time than it would of cost to buy a NEW one. Now I have a 50 1.4 that works but no warranty. The guys at Canon really Suck in the part department. They will show you a bad xerox copy of an exploded view of the lens. Sometimes it is on 2 or 3 pages for one section of a lens. It is hard to see exactly from a email'd copy, of a xerox copy, of a manual, exactly what part one may need.
When you buy parts from Canon they have a No refund policy. Make sure you order exactly what you need (good luck with that). I can un happily say I own 2 extra parts that I didn't need to fix my lens. The parts sales guy on the phone told me what I needed based on my description trying to ID from horrid drawings. Also because the Repair Tech could not cypher the drawings either so he just spit out a Number to get me off the phone.
In the end I did get the right part when I got frustrated and ordered the whole damn assembly. I wish I could get my hands on the lens repair manuals/ microfiche. When you ask to talk to a tech to help ID a part, it is a bad Joke. You end up talking to some know it all jerk that has access to a huge Bin of every part known to man than can ID exactly what they need to fix an Item, but cant help ID anything from the drawings the parts guys use to sell parts to consumers. It is kind of Scamish if you ask me.
They dont want capable people working on their own gear. They want to to send it in so they can charge you a bundle for a few bucks in parts. It's easy to fix stuff if everything is in front of you. Try doing it like I did, the blind stubborn guy method.
Back to the lens. Of the 50's the 1.4 is really over priced. The aperture needs to be 9 blades at least the Bokeh looks like a stop sign and they are all prone to damage from pressure on the front of the lens. PARK your 50 1.4 at Infinity. If you dont you may be very sorry.
Some info I fond trying to help the Parts guys ID what the Heck I want. The Reapir techs wont look at emails of Images either. I tried to take pics of what I wanted, they refused.
I am happy to say I have a fully functioning 50 1.4 but it could of gone a helluva lot smoother and for less Money, if I could talk to someone knowledgeable, and customer service oriented.
I just wanted to share with you my struggle to fix a USM 50. I am going to get the 50L despite the negative tings I have been reading how it performs on a 7D. My Rental 50l will be here the 2nd week in Feb. I cant wait.