That is the mirror lens - Reflex Nikkor. As I recall the 500mm is a fixed f8. Those lenses are either love or hate, it seems; but I've not seen much use of them on digital bodies. Will be interesting to see how you like it - please do post some shots.
JPL wrote:
Well, I bought a 500 mm Nikon for $75 and picked it up today at the post office. Got it on ebay. As it was dark when I picked it up I have not had much chance to try it out, but I got some face pics of my wife that seem ok. I am interested to try it again in daylight. But this lens is fixed aperture and manual focus reflex lens, a bit different from what you are thinking about.