After I bought the Canon FD 135/2 and 200/2,8 I'm looking at adding some even longer focal lengths to my collection. I was originally looking into the Canon FD system for these as well, but they seem to be few in numbers and hard to get. I have more or less decided to go all out on these, and go big aperture, aka larger than f4. Currently I'm looking at 300 and 400 mm lenses, and the Canons (FD 300mm f2,8L, FD 500mm f4,5L) seems very rare in this segment, only a very few on ebay, and they are all too expensive. I'm looking at something around $1500 for the lens itself. So I have started looking at Nikkors. I saw an AI 400mm f3,5 (Predecessor to the 400/2,8) in excellent condition at Keh, the other day, but it got sold rather quickly, but there are still some at ebay. An other option would be the 300mm f2,8 ED IF and a decent teleconverter, which seems cheaper and much more numerous. I see these are available as AI and AI-S, what's the difference? Generally, AI-S designated ones seem to fetch higher prices and are more common than the regular AI designated, despite the fact that the optical formula seems to be the same. So, any of you have any good ideas...?