Issue: Autofocus does not work
Solution: The camera is unable to auto focus when the maximum working aperture falls below f/5.6.
In order to maintain autofocus capabilities, the lens must maintain at maximum working aperture of at least f/5.6 or greater.
If a 1.4x EF Extender is used on a lens that has a maximum working aperture of f/4 or greater, the camera will continue to be able to autofocus. If the same extender is used on a lens with a maximum aperture smaller than f/4, the maximum working aperture will be smaller than f/5.6, so auto focus will not be possible.
If a 2x EF Extender is used on a lens that has a maximum working aperture of f/2.8 or greater, the camera will continue to be able to autofocus. If the same extender is used on a lens with a maximum aperture smaller than f/2.8, the maximum working aperture will be smaller than f/5.6, so auto focus will not be possible.
As taken from smc1377's post on the first page of this thread.
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Rex K
The view from my "office" doesn't suck. 
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