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Re: The Auto focus just stop working
This lens should have a 1 year manufacturers warranty on it, and if my guess that the USM motor is burned out is true I would have it serviced by an authorized service technician. Before you do this though I would try cleaning the lens contacts to make sure it is communicating with the camera body. I of course am going on the assumption that you have tried other lenses on the camera body to make sure the problem is the lens and not the body. I hope this helps!
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there is couple of possibilities like wise when you zoom lens manualy and can you heard the hard noise then could be AF unit itself faulty. AF unit gear is gone.
could be AF shaft broken thats why not focusing
could be AF Flex broken thats why not focusing
for that please bring to canon service center
If your AF motor is hunting then the lens probably needs to be looked at. I wouldn't recommend this as something to do at home so you will need to either find a repair shop if it is out of warranty or return it to Canon if it is in warranty.
hi i am sagha a camera repairer from mumbai, i have gone through such problems much oftenly,in your case i think it is the internal problem with the flex board inside the lens which is multitwisted on metal plate after prolonged use becomes stiff & brittle and creates hairline cracks on the pcb which in turn breaks the tiny tracks & leads to malfunction at 135 mm.
If the AF motor is still alive + spinning, this should be fixable without any replacement parts. Following URLs might be useful: http://www.flickr.com/groups/canon_50mm/discuss/72157603775874649/ And this one - detailed instructions how to dismantle this lens and fix the issue similar to yours. http://www.fotomozaic.ro/artikel.php?idstory=225&s=1
This is because the camera will not expose a picture until the auto focus is locked. If you are shooting in dimly lit conditions, the AF might have a hard time obtaining a lock. Using the flash as auto focus assistance many time helps in this condition.
Another reason might be that you try to focus on even surfaces with low contrast. The auto focus work by sensing contrast. Even surfaces, such as a white wall or the sky, have very little contrast, therefore the auto focus will not be able to obtain a lock.
The solution here is to point the AF sensor at something with more contrast, such as an eye, a pole, or basicly anything that have edges/lines.
I hope this help you!