Can the EOS 80D autofocus when using the EF 100-400mm IS L, and the 1.4x ?

[Waddizzle]

I don’t know off the top of my head. It’s complicated, anyway. I suggest downloading the full User Guide and searching for “lens group “.

[kvbarkley]

It looks like a max aperture of f/5.6 is required, but it *might* work to f/8:

[Waddizzle]

It’s complicated. The 80D shares a similar AF system used in the 90D and 6D2, which can support up to 27 f/8 AF points. Exactly how many varies from one lens to the next, combined with what version of the Canon teleconverter you are using. 
The User Guide has tables to help you determine how many AF points will be enabled, which ones are enabled, and at what sensitivity. 

Like I said it’s complicated. 

[Tronhard]

I used the 80D with the EF 100-400L MkII and the MkIII 1.4x extender.  It did autofocus and I got it to work at f/8 aperture function, but you must be precise about the version of the extender and the lens.  As Bill (@Widdizzle) commented, it's complicated because of the variations of bodies, lenses and extenders and performance varies widely.

Check out this compatibility chart: Canon EF Extenders - Teleconverters (eflens.com)

[Waddizzle]

Correct!  It is complicated and confusing at first. But you still need to reference the charts. 
I believe your Mark II lens will allow the full complement of 27 f/8 AF points to be enabled, but only with the Mark III version of the Canon 1.4x teleconverter.  I believe the enabled points lose their “cross type “ behavior.

Performance with the original lens release just may be reduced to only the center AF point functioning in a degraded mode no matter which Canon teleconverter you use. 

[p4pictures · Accepted solution]

You need to be precise with the lens model and extender model.

If you have EF 100-400mm f/4.5-5.6L IS II USM and EF Extender 1.4x III then you will have cross type AF points with the central 9 AF points, plus horizontal line sensitive AF points in the blocks of 9 points either side of the centre group. In the manual this is group G.

For any other models of extender and the mark 1 "push-pull" lens then only the 1 AF point at the centre will be able to focus, this is Group H in the manual.

[RJU]

Thanks. I have two 80d’s and a 1.4X v1. I was hoping to sell my Sigma 150-600mm C, and get a used 100-400mm markII, and use a 1.4X to achieve 560mm, but I was concerned that my 1.4 was too old to work with the center group of cross sensors. I’ll have to decided if I can afford to buy an additional 1.4X in the mark III version, as part of my plan. I wasn’t allowed to explain this all in the question format. Your answer is spot on. Thanks again.

[Waddizzle]

Great answer. But this barely scratches the surface. You still need to see the illustrations in the full User Guide to understand the bigger picture. 

He referred to @Group H”.  Download the manual and search for “lens group “. 

[JHTibbets]

As a second actual test, I attached the 100-400mm 4.5-5.6L to my old 60D that I keep around as a backup. I presume the AF system is not as advanced as the one on your 80D, but your 80D is unlikely to focus at as low of light as the mirrorless can.

The combination of the 100-400 and 1.4x does not work at all, even in bright light. if it doesn't function on the 80D in real world conditions, I am sure others would like to know. It may also be time to upgrade the body, but you can use EF lenses with an EF to R adapter. I have found all Canon EF and the Sigma, Tokina, and Tamron lenses I have all work at least as well as on the 60D. The bonus was that the 1.4x works on lens combinations that were previously not recommended on any Canon DSLR body I tried mounting them on.