Something you may want to look at is an adapter to go to Nikon lenses from the 80's (nikon f mount I think). No auto focus, but optically these lenses are really sharp. There is a 500mm focal length "mirror" lens that you can get for around 100~200 dollars. It's not a perfect 500mm lens, but the mirror lenses made today are made as cheaply as possible and are nowhere near as good as the nikon lens from the 80's, and refractor lenses in that focal length are heavy and very expensive comparatively.