EF 100-400 Auto Focus does not work.

[ccobb66]

I have an EF 100-400mm f/4.5-5.6L IS USM.  The Auto Focus does not work.  When I turn the camera on the letters MF flashes in the window on the back.  The lens is set to AutoFocus and when I change it to ONE SHOOT or AL SERVO it flashes a message that says "Set the focus mode switch on the lens to AF"  which it is already there.  If I switch the lens to MF - it does not flash.   

[jrhoffman75]

Welcome to the forum.

From your description it sounds like the switch has possibly failed.

Try cycling the switch a few times to possible clean contacts.

Also remove and replace the lens a few times as well.

If that doesn't resolve the problem it will probably need service.

[ebiggs1]

"...  it will probably need service."   Sorry. 

[ccobb66]

Thank you. That is what I thought also.

[ebiggs1]

"When I turn the camera on the letters MF flashes in the window on the back."

This tells you it isn't any contact(s).  It is making connection for that to happen. Hopefully is it just the AF/MF switch. That will be an easy and fairly inexpensive fix. Plus your lens will come back looking brand new. Canon will also check out the rest of the lens to new specs.

[ccobb66]

Thank you.  Sent it off yesterday and I already miss it.  

[Waddizzle]

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@ccobb66 wrote:

I have an EF 100-400mm f/4.5-5.6L IS USM.  The Auto Focus does not work.  When I turn the camera on the letters MF flashes in the window on the back.  The lens is set to AutoFocus and when I change it to ONE SHOOT or AL SERVO it flashes a message that says "Set the focus mode switch on the lens to AF"  which it is already there.  If I switch the lens to MF - it does not flash.   

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Are you using a Canon body?  Those are some unusual messages.  Sounds like you have a non-Canon body and a bad or wrong type of mount adapter, to me.