I putt these in an earlier thread before, both shot with 50D and 70-200 F4L:





This is f4 at 100mm. I should notice that there's at least 15m/50 feet of free space behind the subject and the subject is probably close to minimum focus distance





This is at 70mm f4. You'll see that I needed to use 70mm otherwise I had to stand back a lot further. At 200mm this photo would have looked a bit better. Here you see what happens when there isn't a lot of room behind your subject.