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.