That's what I have been seeing as well subject track off for stills equals sharper birds , I am shooting an adapted 100-400 MII and am also finding lens IS off seems to increase detail/ focus as well. I have not tried the opposite camera IS off lens is on. My testing has been with backyard birds. And I did see what the Op with the Hawk seen only with A juvenile eagle that was launching from a tree . The 5th shot of the series the head eye and beak sharp, Talons and legs blurred and shot at 1/1600 ss . I am not complaining and over the moon about this little gem. I have so many shots that are keepers that I don't even do post on because another just has a little better look to it. In fact if I had any complaints it would be I wish I could go 10 fps electronic instead of 15 (Shooting H) because I am taking to many dang shots.


