May I squeeze a question in here:
Can I MA manual focus lenses? Dones this make sense - I thought the MA corrects the lens USM where to stop...
As the others have said, YES if it has an auto confirm chip on it.
I have the Rokinon 35mm f 1.4 manual lens and added an autoconfirm chip and it did indeed need AFMA on both 5D2 and 50D to get the focus light/beep to trigger on the deisred focus plane. And even though the Rokinon 35 doesn't have this problem, a manual lens with focus shift will be as much a problem using a chip as a full auto would be.
Once I did this, the lens becomes in effect "semi-manual" or "semi-auto" (take your pick) since it can be shot in AP - you just manually adjust f-stop on the lens and then set the body to the same f value, and auto exposure now works. Then just press the shutter half way and twist focus until you get the usual red light and beep and after AFMA, very sharp focus is achieved.
Full manual would be a real handful without the "semi-auto" features...a definite must-have especially when you can get the programmable version af confirm chips for under $20 on eBay. Basically your hands become the motors for both focus and aperture and the camera body does the same things as usual with the chip installed on lens.