SOURCE: lens auto focus sometimes does not work.
First make sure contacts on lens are clean ( see enclosed pic #2 ) . If this does not solve the problem the ultrasonic motor may be faulty pic #1.( sometimes switching on but sometimes not , will have to be replaced , unless there is a bad connection between the motor and lens pc board )
#1 #2
SOURCE: EF-S 17-85mm will not auto focus on fuul zoom my
A dirty filter or smudges/marks on the optics, especially the rear element can cause focus issues. Cleaning the pins contacts on the mount of the lens can be a simple solution too. Beyond that, circuit replacement is the next likely step.
SOURCE: 28-80 mm sigma won't auto focus
The lens may not be a dynex type so it wont work on your 700
SOURCE: Auto focus not working?
1st u reset the factory setting and then u see the lense is correctly fitted or not and the autofocus option is working or the manual focusing is there or not
SOURCE: Canon 17-85 is usm lens very loose focus ring causing lens creep
I also have a 40D with exactly the same lens and fault. Over the course of a few months the lens became "sloppy" so if the lens was pointing towards the ground it began to creep. Towards the end it dropped to its full extent. About 10 days ago it locked up completely and I was unable to zoom at all. Returned to Canon as mine was just in warranty and they have fixed and returned it within 7 days. Very effecient even sending me a text message to say that it had been dispatched. The lens is now just like new and I only hope that this is not an annual trip.
Bottom line; I am an engineer by profession and I wouldn't touch it with a barge pole. To do these properly you need special tools and jigs to make sure things line up properly. It also needs to be done in a "CLEAN" environment.Do yourself a favour and leave it alone.
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