I can confirm the AI and AIS 16/2.8 are unit focusing lenses, they don't have CRC. The AF-D version has a very similar optical design, to me it clearly is derived from the AI optics but it now has CRC, permitting the focus range to be shortened from 0.3m to 0.25m.
All Nikon 16/2.8 versions require a rear filter to be fitted for normal photography. Without the filter the lens can focus closer than normal, but cannot focus to infinity.
Performance in the center is very good, but the corners only become acceptable on stopping well down, for landscapes where corner to corner sharpness is required I usually shoot at f/11. According to some reports the Sigma 15mm fisheye does better here - if you find a good copy.
The 16mm has a relatively long focal length for a lens with 180° field of view - the Sigma and Canon fisheyes have 15mm focal length with the same field of view. That means the central portion of the image is magnified relatively more, with the edges highly compressed more to squeeze the same field of view into the frame. The result is higher distortion and compressing of objects at the edge of the frame, which I sometimes find is unpleasant. The 15mm fisheyes, or the Nikon 16/3.5 fisheye with the smaller 170° field of view should have more even distortion characteristics across the frame.
There is also the Samyang 12mm fisheye which uses the stereographic projection instead. The shorter focal length means the center of the image is smaller, and the edges are less compressed resulting in a milder fisheye effect and less distortion across the frame. This lens is relatively inexpensive, I've been quite tempted to get one but reviews are mixed, there seems to be a lot of variation between samples...
