Will autofocus work if I use an EF 1.4X III Telephoto Extender with a EF 500mm f/4L IS II USM Lens ?

[TTMartin]

The resulting aperture will be f/5.6 so yes it will work with all Canon EOS cameras and all AF points.

[ebiggs1]

AF may be your least worry!  If you are using a cropper you are talking some serious focal length.  Over 1000mm !  It will become very difficult to get sharp pictures.  A good sturdy tripod is necessary.

[Waddizzle]

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@ebiggs1 wrote:

AF may be your least worry!  If you are using a cropper you are talking some serious focal length.  Over 1000mm !  It will become very difficult to get sharp pictures.  A good sturdy tripod is necessary.

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Ernie raises a good point.  If the AF does not work well, with or without the extender, it is because the camera's field of view is moving around too much for it to get a focus lock. 

To get an idea of what I am talking about, mount your longest focal length lens onto your camera, and then mount your camera on a tripod.  Put the lens into MF, switch the camera to LiveView focusing on the LCD, magnify the image to the maximum, and now try to focus the lens. 

Do not be surprised if the image shakes around too much to focus the image.  Half the problem could be the tripod, and the other half the tripod head itself.

[EastLelandDavid]

The lens and extender should AF, lighting and other factors will effect how quickly it happens.  You might try using live-view initially but the f/4 x 1.4 will not create problems. Good luck.