Picture taken with Nikkor 6mm f2.8s (Image by talktoher)
You are looking at one of the most gorgeous looking lens in 35mm SLR photography - a lens that can actually see behind itself! Introduced in 1972, this lens was not just an upgrade from the Non-Ai Nikkor Fisheye 6mm f/5.6 lens but it was completely being redesigned optically. The major difference between the two versions is, this lens has a retrofocus design which does not required mirror lock up in the camera section to work. It enables direct reflex viewing and metering through the lens. The Ai-S version was introduced only in early 1983. Optically, both lenses remained unchanged in its optical composition other than went through a transitional changes which occurred in 1977 and Ai-S during early the '80. One of these fisheye lenses actually sold on ebay for $75,000 in October 01, 2013. Here's some photos of Nikkor 6mm f/2.8s, via auctiva.
(Infomation from MIR, via Lomography)