Quote Originally Posted by Sabyasachi Patra View Post
When you use the 100-400 lens with a 1.4x TC, the aperture becomes f8. So you will lose auto focus function with all cameras except the 1 series cameras. In the 1 series cameras (except the yet to be launched Canon EOS 1DX), you can use the centre AF point to auto focus. If I remember right you were using a 7D. In the EOS 7D camera, you will lose auto focus and have to manually focus if you use the 1.4x (I, II, III versions).
It works as follows:

http://www.michaelfurtman.com/taping_the_pins.htm