Guys, I am thinking of buying a Nikkor 500mm f/8 reflex lens secondhand (the "newer" model that focuses down to 5'). I have read that it is a bit^% to focus and hard to get a sharp shot. Does anyone have any experience of using one. I am looking to pay about €200 for one and use a D810 and F5 bodies. My oft asked question - am I mad?