The r5 autofocus for animals, while it could be better, is still very good. It was able to correctly identify the eye of the Kestrel in this picture, however, when using the EF 2x III TC the focus was very soft as seen in the shot below.
I was using a tripod, there was wind, so I turned on the stabilizer and used mode 1 or 2 I can't remember, which specifically.
I was pretty disappointed with this because it had just caught some pretty and had the focus been sharp, it would have been a good shot. Other shots without using the TC came out just fine and very sharp focus (thanks to the 400mm f/2.8). Any advice? Settings I am missing? that could help with this?
The TC doesn't seem to be as sharp even when being used with the 400mm f/2.8 in well lit conditions.
Shutter: 1/2500th, f/8.0, ISO 800
I hope some of this might be helpful.
The 2x teleconverter will reduce the contrast for small features. The air at a long distance will have varying densities which cause the air to act like a distorting lens. For me, since I do not have an F/2.8 prime, there is also increased small aperture diffraction blur, but that will not be a problem at F/5.6.
But, I often use EF100-400mm f/4.5-5.6L IS II USM +2x III which is F/11wide open.
With the EOS R5 and IBIS, there is no need for such a fast shutter speed unless the bird is moving. For my Kestrel, I stopped down to F/13 with F/11 wide open. It was hand held with IBIS enabled. Ernie tells me that this photo is not up to his standards, but I like it. I set the camera to save DPRAW and used Canon DPP to adjust the focus distance +1 with strength of 8. I set digital lens optimizer to 65, mostly because of the small aperture. The combination of digital lens optimizer and DPRAW tool overcome most of the disadvantages of the 2x III. I also put the camera into 1.6x crop mode because it seems to me more likely to find the eye of the bird than in full frame mode. In DPP, the DPRAW tool will overcome some of the lack of contrast for small features, but with enough distance, there will still be blur.
https://www.rsok.com/~jrm/2024Feb09_SaltPlainsNWR/2024feb09_kestrel_IMG_8938c.html
https://www.rsok.com/~jrm/2024Feb09_SaltPlainsNWR/IMG_8938cs2.JPG 
@ebiggs1 wrote:"Another contributing factor is you were clearly a long distance from your subject ..."
I have to agree with Rodger as I almost always do, I might add. You are too far away. Closer is always better than any great lens. There is no cure photographically for poor "seeing" conditions. I also have tested several 2x tel-cons even several form other brands and have found they all to be wanting. They are simply not good if great IQ is your main most thing. Now of course this is qualified by what your personal standards and goals are. What I accept or what Rodger considers OK isn't important; it is what you are good with that matters.
Not only personal standards and goals, but also budget. The 800mm prime is clearly better than the 100-400 with 2x.
https://personal.canon.jp/product/camera/ef/extender-ef2-iii/spec
https://personal.canon.jp/product/camera/rf/rf800-f56l/spec
The RF 200-800 might be the best low cost option to get to 800mm now: https://personal.canon.jp/product/camera/rf/rf200-800-f63-9/spec
@ebiggs1 wrote:John that's all fine and good if you intend on shooting pretty graph paper. The truth and final judgement is in the product that the combo provides for the photographer. If what they get is what they are good with fine. It is not for me. The one bird example above is soft, more soft then I like to see for instance but the shooter is good with it so, so am I.
Another example at a shorter distance. It is easier to get a sharp image at a shorter distance. Is this image too soft for you? It is softer than it would have been with the $17000 800mm F/5.6 lens which is too expensive for me. "EF100-400mm f/4.5-5.6L IS II USM +2x III" is a much less expensive way to get 800mm focal length. At a focal length of 400mm and the same distance, there would have been less detail and only one fourth as many pixels on the bird. The image seems to me sharp enough to identify the bird and document the sighting.
