R5 Mark II is not properly focusing on birds eye

[Lanner]

Recently as I’ve been attempting to photograph birds, I have been noticing that my R5 Mark II struggles to correctly focus on the eye of the bird, and sometimes the bird itself. To my knowledge, I have all of the correct settings in use (based upon multiple YouTube videos and google searches), yet still struggle to get good focus. I am using a Sigma 150-600mm Contemporary lens with the EF to RF lens converter with focus ring, so range is certainly not the issue. Even though the bird itself tends to be in focus, the eye is just not consistently in focus and as sharp as other parts of the bird. 
I use eye tracking autofocus and whole area autofocus typically with servo. Usually, in the viewfinder, the eye of the bird is in a blue box, but I still feel like the images are not in focus that well. Any suggestions? Thanks! 

[Lanner]

Forgot to mention, I am not currently on newest firmware and I am updating that soon so will keep you guys posted 

[TomRamsey]

On my R6II is is not always easy to focus on a bird deep in a tree even with a clear shot, the camera will sometimes want to focus on a closer limb.  I have found quickly manual focusing to get close and then autofocusing will work.  But it doesn't sound as if that is your problem.  That eye should be very much in focus.  Are you sure it is focus and not camera movement, what kind of shutter speeds are you shooting?

[Lanner]

The shutter speed is like at least 1/800 so I dont think it is that. The birds are also stationary and not too far with zero branches in front of them. The camera will show a blue box around their eye and while I am holding the AF button I click the shutter and yet it is still not in focus.

[TomRamsey]

Sounds like you are using back button focus, and I'm not sure if you can also have the shutter button focus but if so that could cause a problem.  I'm guessing you can't do that, but just looking for possible problems.  I know a big part of the reason BBF is so effective is because it keeps the shutter button from changing focus.

[Waddizzle · Accepted solution]

“  I am using a Sigma 150-600mm Contemporary lens with the EF to RF lens converter with focus ring, so range is certainly not the issue. Even though the bird itself tends to be in focus, the eye is just not consistently in focus and as sharp as other parts of the bird. 

I use eye tracking autofocus and whole area autofocus typically with servo. “

The lens is the issue. It wasn’t designed for Dual Pixel AF image sensors. A firmware update will not correct all the hardware deficiencies. 

[Waddizzle]

“ The shutter speed is like at least 1/800 so I dont think it is that. “

Sure it can. I strive to shoot at least 1/1600.

[Lanner]

This is very helpful, thank you.
Going off of that point, im now inclined to upgrade lenses.
For wildlife, I dont think 400mm will be sufficient, so im kinda left with minimal options, such as the 100-500mm. I don't know much about the super telephotos that would be best compatible with my camera (and not cost 13k), so do you, or anyone, have any suggestions? 

[TomRamsey]

I am extremely happy with my RF 200-800, the lens that drove me to Canon in the first place.  I had never imagined that I would be able to afford an 800mm lens that was this good, and also be hand holdable.