Last May, i rented a 500mm lens to shoot the Atlantis Space Shuttle - and on the way home I wanted to "use" it, so we pulled into the WPB zoo. Never again. Never again will I rent a 15LB lens! He weighed too much for me to handle - mounted even, I could not carry it around or use it comfortably. My D700 with my 70-200 is about as much weight as I want to carry comfortably. I commend you folks that can carry around those lenses, because you are getting the shot!