A couple of weeks ago I went to the lake in Boiling Springs to get some panning practice with the 200-400mm lens. From what I read about the lens before I purchased it many referred to the lens as a hand held unit. That may be true and these were taken hand held, but you do not do it for very long before needing a break. I guess if I use it long enough I will get some guns for arms for the first time in my life. I would like to hear from any one else that has this lens and has used it for flying birds but here is my take on it. It does bring the subject closer and that is what a big lens is for, with the f4 it allows me to get my shutter speed up without increasing the ISO to high. But I felt that it was harder to pick the bird up initially and slower to focus on the subject and keep the focus like the 70-200 f2.8. Now this was only the first time I used this lens for this type of photography. To be honest I would like to shoot it for about a year before I really make my final decision on the lens.