The d-o-f at 80-feet is (as you said) 1-foot, but at f/10 it is 75% more (1.78-feet). That's not small!
At f/10 more of the bird would be
That's quite a different subject.
Song birds in flight at close range is a very difficult subject with a 600mm, it
Arash, eventually I hope to have more songbird pictures; both in flight and perched. I am particularly interested in warblers.
I would assume that