Interesting Spokane, I was not even thinking about the fact that the camera eyepiece is not ideal for the purpose of using as a scope therefore needing an adapter. Thanks!
To clarify what I am looking for from the forum community is: those who have used super teles above 500mm, do you feel you see enough detail that you could discern a pencil sized hole in white paper?
Using viggen61's rough estimate formula I am assuming 600mm is going to be the minimum, but I would like some real world confirmation? If this is correct, I can add a 2x extender and the eyepiece adapter to the 100-400mm and be good in theory...