If you got what some call "pumping" or "hunting" focusing, the lens tries to focus but
seem to be unable and ends with a clearly unfocused image. Then your lens is dirty inside.
You can fix it yourself - BUT...DONT
DO IT, IF YOU HAVE TEN THUMBS, If you break it - i
warned you right here - i
am not to blame - DONT
kind of hard to describe, but i
will try anyway.
If you have disassembled your lens so much that you have the usm part as a separate unit, then try an have a look inside the usm tube to see what happens when you manually change the focus.
Inside the tube there is a small slit where the optical sensor is
hidden, you can\'t get to it without removing the rear part of the usm unit
. This is tricky, because one of the flat cables is taped to the
inside of the tube. You\'ll have to gently remove this cable from
the tube - WARNING: these cables are very sensitive to pulling, they
will break if you pull it too hard. (This is probably why Canon Service Centers are replacing the complete USM unit, instead of trying to repair the darn thing)
After the rear part has been removed, you should be able to see the
plastic/glass sensor plate about a 1/4 of the diameter of the tube.
DON\'T touch this plastic/glass thing with anything hard, use a fine (new) paint brush to clean it. If you have a magnifier about 5x to 8x, you should be able to see that the sensorplate is dirty.
Try to move the sensor itself (not the plate) by manually focussing, so that you can clean the plate beneath it.
put the lens back together, it worked wonderfully with my lens. Will it work with yours? don\'t know.
And as always, you are on your own, if you break it - your problem, not mine!