MatthewK wrote in post #18968187
Today I finally determined exactly why the EOS R's AF isn't suited for my birding photography (600 III and w/wo 1.4 TC), which most often occurs in brushy/wooded areas where you have complicated backgrounds. I was going to make a quick video, but I didn't have a tripod, so use your imaginations for now
For this example, pretend there's a clear shot of a bird on a branch ~10 meters away, nothing in between me and the target, and the background consists of other trees/branches/vines/etc a few meters further (like my Cardinal shot here). If I start at MFD and AF on the bird, the camera locks on with no problem... but, if focus is well out past the bird and I try bringing focus back in, the camera sticks to the background, and no amount of pumping the AF-ON will convince it otherwise. The only thing that changes its mind is racking the focus ring back to MFD or closer than the target, and trying to AF again. Thus, if I missed focus or happen to be focused out further, most likely I have to intervene in order to ensure focus, and by the time I get done fiddling around, the bird had moved or left completely.
This behavior caused me to miss a lot of shots this past fall while shooting small warblers up in the trees, where there were too many opportunities for the AF to hit on something unintended while trying to track the birds.
Since I now also have the EOS-R for birding (with the 400DOII+extenders), I will be dealing with this also, although it is not new to me. I have been shooting the 400DOII with extenders on the Sony A7RII by means of the sigma mc-11 adapter, and it has also been an issue: manual aid necessary to get the focus in front of the subject, because when past the subject, it will not find focus anymore.
This is a programming issue mainly I believe, because firmware updates on the Sigma MC-11 as well as on Sony bodies other than the A7RII (which no longer receives any support) have greatly improved things, and it appears that the 400DOII is now "as native" on the Sony A9 with the latest firmware.
So for those hoping to use their EF big whites on the EOS-R for a bit longer, it will depend on how much Canon can improve on their sensor based AF, and how much of this improvement they can slipstream into the EOS-R. Personally, I decided to go for the EOS-R over a Sony A9, because I have not come to grips with the colors of the 400DOII on the A7RII, and have been getting some lens aberrations in bright light that make me think that the lens is not tuned properly to the Sony sensor cover glass. Color is one of the most important things in birding/wildlife, and I decided to leave my Sony A7RII for the Loxias, that dó produce great color on the Sony body. Your shot with the 600mm does have great color also i.m.o.