Re: Picture sharpness on Canon M6 ii with Tamron 100-400mm
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Based on the photo you have posted I agree that it does not look up to standard at all. It is very soft.
This could be an issue with the lens, or it could be out of focus. With almost any telephoto lens + moving subjects, finding & keeping focus locked on to subjects is difficult. You then also have to take into account your f-stop which at the high focal lengths your depth of field gets quite narrow so when the focus moves just a tiny bit it can be enough to throw your whole subject out of focus. It is essential that you are shooting in burst mode to account for this as it is normal for the autofocus system to hunt around for the best focus when the subject is moving - I typically shoot in low-speed burst unless there a specific action moment I'm trying to catch then I use high-speed burst. No matter what, you will end up losing shots due to this and it happens even on the high end gear - Duade Paton shows on a couple of videos that even on the R-series with the super fast bird eye AF, using the best available lenses on both the R5 and R7 and a tripod, you will still end up losing a few shots while the focus is hunting around.
Here is an example of this - these photos were taken ~1 second apart with no input from me, just holding the button and hoping for the best Can see the first one is way out of focus and looks similar to yours, but the following one is nice and sharp.


Of course, if you were already taking all of this in to account and that image you posted is still the best example of it being in focus, I would be very inclined to say it may be an issue with the unit.
Regarding your issues with tracking - when you are far away with a small subject like with the Kingfisher you should probably be using spot or single point AF rather than the default 'tracking' mode as it is not really designed for subjects that are small in the frame - I find it is great for general use but for the really small stuff you will need to switch to a different mode. Generally speaking I find the AF great, one of the reasons I went all-in on Canon.