Most of the thoughts I posted relative to the D800/810/820 and the D500 have relevance to this new fisheye lens as well as other lenses. And were made with the idea that if someone is thinking they might want to buy a D500 to get the "wider" zoom range of 180 degrees to 110 degrees that this lens provides on a DX camera, which is similar to the Tokina 10-17mm lens, that they could get the same zoom range by switching their FX camera to the DX crop mode and lose very little in comparison to a DX only camera. There have been many posts concerning the Tokina 10-17 which is a DX only lens unless you use it with a teleconverter which degrades its image quality. My feeling is that if you shoot a DX camera, and want a similar zoom range, the Nikon lens will provide this with considerably higher image quality.
I did not shoot any pictures underwater at 8mm in the FX mode. Everything I shot was at the 15mm (full frame fisheye) setting. As I wrote initially, I am not interested in circular fisheye images and I would have needed a zoom gear to make any changes while underwater, which I did not have at that time. My interest in this lens is because I can shoot it behind the wideport at 15mm with no vignetting whereas I cannot do this with the Sigma 15mm. And because it focuses so closely.
I had Nikon's own lens shade mounted on the lens when I took the picture of the octopuses that I posted. I shot similar images at f11, f16 and f22 and saw no vignetting in any frame. Since the lens is an internal focus design, that means the angle of coverage of the lens gets slightly wider as you focus closer and closer and the octopuses were only about six inches from the from of the wideport's glass surface.
The lens was behind the Seacam wideport with a 20mm extension ring between the wideport and the housing body. If I get some free time I will take pictures of the housing with the port and extension ring in position as they were during the dive and post them. I will also post a couple additional pictures that show that whereas the Sigma 15mm lens almost always exhibits some minor vignetting with the wideport, even without any extension ring, the Nikon lens does not.