I don't own any extenders but I was considering the Canon 2X (II or III) - I had heard tales of it losing AF and wanted some advice. In a previous post on POTN I was told it loses AF at 400mm, but I can see from the discussion here that it's about the aperture not the focal length. A TC may work better at the minimum zoom than maximum, but I'm sure no one bolts on a TC to shoot at 200mm with a 100-400 zoom.
My body is a Canon 5D mk IV.
Sounds like 1.4X III is the one I should be looking into.
Thank you all.
That's a good first step. You might be more inclined to use it than a 2x. A 2x needs a faster lens to AF well (if at all), and if you want AF, a 1.4x will work with more equipment and circumstances.