100-400 ii and 1.4x, difference between ii and iii?

IMO the difference between using the 7D2 vs the 5D4 and an extender comes down to the ability of the lens system to actually use the higher pixel density of the 7D2 vs the 5D4 plus the lighting situation, combined with your own personal preference.

All of these cameras (7D, 7D2, 5D4, 1DX) are complex electromechanical systems that require the operator to work with in order to achieve the best results; you can't just slap a long lens you aren't familiar with onto a body you aren't familiar with and instantly get the best possible results. The 5D4, with a given lens, puts 60% of the 7D2's pixels on target, but those pixels come from a 3 year newer sensor coupled with a newer AF system.

Its going to come down to user preferences.

I was happy with the 1.4x II when using the 7D2 with 70-200/F4 IS, but I did not like it when I attached the 100-400II. It was good, but something was not exactly what I wanted, shooting sitting birds on branches and cropping the shots considerably. A fellow photographer recommended to try the 1.4x III, and it was noticeably better. Now the II is used by the wife on the 70D with the 70-200.

Lbsimon wrote in post #18307265
I was happy with the 1.4x II when using the 7D2 with 70-200/F4 IS, but I did not like it when I attached the 100-400II. It was good, but something was not exactly what I wanted, shooting sitting birds on branches and cropping the shots considerably. A fellow photographer recommended to try the 1.4x III, and it was noticeably better. Now the II is used by the wife on the 70D with the 70-200.

7D2 allows f/8 AF, but its performance is marginal compared to state of the art - you need contrast and light to avoid hunting, and AF is only snappy for relatively small changes in focus depth. Like someone said earlier in the thread, the 7D2 came out just a little to soon to get the latest f/8 AF technology.

Now, the difference you see between vII and vIII, is it mainly optical, or is it AF confidence? Of course, a poor MFA could provide a confident, but slightly-off AF, which could seem to be an optical problem.

I did not detect a difference in AF speed, but rather the rather in the sharpness. The camera is AFMA'd for that lens plus the 1.4x TC's, both II and III.

CyberDyneSystems wrote in post #18305565
No, it doesn't,. but that's a matter of opinion.

The reason I shot the 1D series exclusively for 10 plus years over any 1.6x with higher pixel density holds true with the 5D4 as well,.. better AF and better IQ trumps pixel density.

This of course is my opinion, but it is what I believe and that's how I've been doing it since the EOS 1D.

This has been my experience too. When looking for reach my 7D2 can outperform my 1DX - but not often! Under ideal conditions the higher pixel density of the 7D2 can be an advantage but, unless conditions are very good, I don't even bother bringing it along! In my experience the "Reach Advantage"/"Pixel Density" of the 7D2 gives (under ideal conditions of light/AF requirements etc) and increase in range of maybe 1.2 so at 12 meters it would match the 1DX at 10 meters or so. Far less than one would expect from the densely packed sensor of the 7D2 compared to the 1DX Pixel Desert and then only under very good conditions!