This incredible beauty is an underwater fisheye. Made for the first underwater interchangeable lens reflex system, the Nikon RS.
This was designed for underwater use, like all Nikonos lenses, with very few exceptions. It was the main selling point of the Nikonos system; every lens was designed with the last glass-water surface in mind, avoiding domes, points of failure, and optical compromises. That means no land use, sorry. It would simply not be able to focus, would have poor field curvature, and an excess of aberrations. As for the design, it is adapted from the Nikkor 16mm 2.8, and shares also the bayonet filters at the rear, but covers 170 degrees on the diagonal instead of 180.
Nikon original patent, the US version, is 5579169.
And here we have the turning point: this 30-year-old gem was modded and rendered usable in many ways:
Here by a clever guy named Isaac Szabo.
https://www.isaacszabo.com/13mm.html
The great ingenuity of this mod was to re case the whole inner element in a Sony mount lens while 3d printing a dome-like housing, where the viewport IS the massive 13mm Nikkor dome-shaped first lens (a proper LENS with double different curvature, as seen in the optical scheme. The full contraption is then mounted on a Nauticam Sony housing as it was any other lens.
Here is the mod by Borut Furlan:
https://www.borutfurlan.com/articles/nikonos_RS13.html
clever article about this lens and the possibility of recasing it in Nikon DSLR form (commercially available from Seacam)
https://www.seacam.com/en/nikonos-rs-conversions/
a great THANKYOU to Marco Cavina for the infos, you can find far more on his NIKONOS PAGE.
As always MIR pages are a precious information source
