The 16 mm Fisheye-Nikkors are designed to fill the entire FX frame. The DX equivalent is the 10.5 mm f/2.8 AF-Nikkor, likewise covering the entire DX frame. The various 6 to 10 mm Fisheye-Nikkors do render a circular image on FX, though.
If the 10.5 is put on an FX camera and its hood removed, it will cover about 200 degrees on the long axis. Still, top and bottom will be masked off, but by quickly rotating the camera for another exposure and later combining the two frames, a circular image results.
The image below, from the Dutch city of Delft, is taken using this two-frame technique.