I'm new to Canon Rumors and so glad I found it because it's been a great source of knowledge for me. I recently bought a 400/5.6 mainly for birding and wildlife photography. I chose that particular lens because of its relatively low cost compared to the other super telephotos in Canon's lineup. I've been thinking about getting a TC to use in combination with my 7D and 400 for the extra reach but am concerned this combination might not work because of an issue with F8 center point AF I've seen mentioned before. I don't fully understand what this is about but would like to be sure that the TC will work before I spend the money on it. Any enlightenment on this topic is greatly appreciated as I may be totally off base in my understanding.
With that combination, the 7D cannot autofocus using the viewfinder and the 19 AF points. Putting a 1.4x Extender on an f/5.6 lens makes the maximum aperture f/8, and the 7D only will AF down to f/5.6 max aperture.
The 7D can use Live View Autofocus with this combination, however. You mount the extender to the lens and the lens/extender onto the camera (and hopefully to a tripod!). Then turn on Live View with the button on the rear near the viewfinder. You will see the scene with a white box. Put the white box on what you want to focus on, and then engage the AF, and the lens will focus, though very slowly.
You can "zoom in" on the image to assist in the AF process.
I use the 1.4x III Extender with my 100-400 f/4.5-5.6L IS USM, on my gripped 7D, and it works ok. That combination requires a very steady platform (cheap department-store tripods need not apply) to work from.