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Usually troubles with autofocus are related to a non working AF. I've got the opposite issue: AF is working too well. Actually, it is ALWAYS working, and I cannot turn it of.

Even when I switch to Manual, when I half-press the shutter button AF kicks in. The lens can be manually focused even with AF on: if I keep the button pressed and defocus manually, it autofocuses again.

I've got two alternative fixes:

  • with the "usual" set up (shutter button focuses and meters) I can press AF-ON button to stop the autofocus,
  • or I can set the custom function IV-I to 3 so that the shutter button does the AE lock and the shutter actuation and the AF-ON button focuses and meters.

I have adopted the first solution which is more in line with my habits, but when using a tripod it can be incovenient (since I'm not pressing af-on and I would like my manual focus decision to be respected).

I strongly suspect that the switch is in some way broken, since otherwise the fixes would not be able to work around it, but I thought to ask here for advices/suggestion/ideas.

If it matters, the camera is an Eos 50D and obviously with other lenses the switch is respected (namely the 70-200 L, which similarly allows manual focusing when AF is on).

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I am quite positive the AF/MF switch is broken. This has happened to a few friends of mine as well and can be fixed easily. If your lens is still under warranty, take it to a Canon service center.

Though the other similar cases I have heard were mostly third party lenses (2 Tamrons and one Tokina) and it's a bit unusual for a Canon L glass, but, as I said, it should be an easy fix (or at least we should hope so!).

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  • \$\begingroup\$ Thanks, I've never been to the local Canon service center, tomorrow I will call them. No warranty, though :-( \$\endgroup\$
    – Francesco
    Jul 8, 2012 at 7:54
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    \$\begingroup\$ For out-of-warranty work, usually they will give you an estimate first, and you can decide whether to get the work done or not if it's too expensive. I've done this with several lenses and cameras. Usually the fees are surprisingly reasonable. \$\endgroup\$
    – djangodude
    Jul 8, 2012 at 15:52

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