I feel that the 400 DO is far superior to the older 400 f/5.6 for handheld flight photography for two very obvious reasons: it is a stop faster and the image stabilization makes a big difference in sharpness at least for me. The few times that I have gone back and tried to use my old “toy lens” it seems that the lens is jerking around all over the place as I attempt to frame the image: I am unable to hold the lens as steady as I used to even when tracking moving subjects. I will admit that I have been doing so much flight photography here on Midway that at time my left wrist does hurt! For folks with a steady hand and a smaller budget the 400 f/5.6L lens can be a great flight lens. On the second Midway trip well more than half of the group has both the 400 DO and the 70-200 f/4 L IS and we are all using them ALOT!