I have the above lens and until recently it worked perfectly. Unfortunatly it now has an unusual fault across all three of my cameras 1DS3, 1D4 and 7D. My usual technique was to focus manually or focus as close to the subject as possible then autofocus. This is the worlds longest time to focus lens, it just takes quite a while. A 500f4 it isnt. Anyway, it used to auto focus perfectly. Unfortunatly when i try to autofocus from any distance, the orange light in the viewfinder comes on telling me the glass aint moving and it wont autofocus. You can see the glass trying to move a little back and forth as it struggles to move but nope, move it wont. You can get it to autofocus by manual focus very close to perfect then it will judder and eventually jump into focus at which it then takes a pin sharp shot. Just wondered if anyone knew the cause before it winds its merry way to C.P.S. Wonder if its the motor? A guy i know who fixes lens has three 24-105's in due to internal breaks. And Nikons no better. But at least he can get spares for the Canons. He cant for the Nikons.