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I bought a Canon EF 28-105mm f/3.5-4.5 II USM lens for my Canon 350D. It works mostly fine, except after switching from AF to MF, I can't return to AF mode.

After I switch MF->AF, the lens doesn't activate the motor. Half-pressing the shutter will not result in focusing, the focus LED will be lit only if I point the camera at something at a distance it was manually focused for. Interestingly, the aperture works okay, I can see it opening and closing in Av mode as I rotate the turnwheel and hold the aperture preview button.

The way to restore the autofocus operation is either to remove the battery (normal on/off switch won't suffice), or to press the lens release button, rotate the lens a little, then snap it back to normal position - normal autofocus activity resumes.

Did anyone encounter such a behavior? Any solutions or suggestions?

(The lens was bought second-hand, and for a very bargain price, so I don't consider the fault a showstopper or a cause to return it to the seller, but I'd still prefer to have the AF/MF switch to work as intended.)

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    \$\begingroup\$ Have you tried cleaning the electrical contacts on the mount and the lens? That may not be it, but is always a good first thing to try. \$\endgroup\$
    – mattdm
    Sep 8, 2011 at 11:13
  • \$\begingroup\$ Some Canon cameras allow you to disable the lens AF/MF switch - check the Custom Function setting IV and make sure nothing is set to AF-OFF. \$\endgroup\$
    – ElendilTheTall
    Sep 8, 2011 at 11:19
  • \$\begingroup\$ @mattdm: I did clean the contacts, no effect. Besides, would removing the battery affect it if it was the contacts? \$\endgroup\$
    – SF.
    Sep 8, 2011 at 13:11
  • \$\begingroup\$ @ElendilTheTall: The option is AF/AE lock, the default. No MF in the options at all. Also, the switch on my kit lens works properly, the problem only appeared with this new one. \$\endgroup\$
    – SF.
    Sep 8, 2011 at 13:14
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    \$\begingroup\$ In that case the next step is to try that lens on another 350D and see if it has the same problem - if it does it's the lens, if not it's the camera. I suspect the lens personally, but it helps to narrow things down. It sounds like the lens AF switch isn't communicating its status to the camera properly, perhaps something wrong with the contact on the AF side of the switch. \$\endgroup\$
    – ElendilTheTall
    Sep 8, 2011 at 14:27

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I have a lens that behaves like that, but when I half press (and hold) and release button and adjust the focus ring, sooner or later, the AF will come back.

Not sure whether this will work in Canon, work in Nikon.

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