F/11 at a distance of 400ft. How good is your image if you select infinity focus and then switch to manual focus? I'm not used to these super long focal lengths, but I'd imagine your DOF is big enough.
I was thinking point it at a nice cloud, autofocus and then select manual focus.
You have to be MUCH more precise than that with long focal lengths. 400 feet really isn't very far, when it comes to DOF at 800mm. At only 400 feet, if two eagles are in the same tree, it is very likely that you can take a shot in which one eagle is nice and sharp, and the other is unacceptably soft.