Quote Originally Posted by Jeff Cashdollar View Post
Sorry Artie,...I am asking, what is a typical subject distance for a medium sized bird (Cardinal). For example, with my 400MM and my 1.6 40d crop factor I need to be within 15/22 feet of my subject to get a decent image not requiring a crop of 30% or larger crop. Just trying to get a feel for distance with this kind of glass , make sense.
I cannot give a definitve answer. For a vertical cardinal, I can probably fill 75% of the frame (the largest that I prefer to go) at about 25+ or so feet with the 800 alone and the Mark III (1.3 crop factor).

When you go from one telephoto to another with a different focal length it takes a while to get the distances down right, i.e., when to stop when you are approaching a subject. Remember: the size of the subject in the frame is a factor of the square of the focal length. So going from a 500 (5 squared = 25) to an 800 (8 squared is 64), the bird will be 2.56 times larger in the frame (64/25). Often times I walk up to a bird with the 800 intending to create an image of the whole bird and find that I have to back way up... Hope that that helps.