I know what may be the reason why Nikon 16mm and TTArtisan 11mm have the same field of view. The lens was designed for Mirrorless and was adapted for the DSLR (I use a Nikon FX camera). That's probably why the field of view is narrower and the same effective focal length. Since you can't actually adapt (with an adapter) lens from Mirrorlens on DSLR (because of flange distance). And they didn't create a completely new lens. They probably keep the numbers for marketing reasons even though the focal length is probably more likely 16mm (for Full Frame DLSR).

I know what may be the reason why Nikon 16mm and TTArtisan 11mm have the same field of view. The lens was designed for Mirrorless and was adapted for the DSLR (I use a Nikon FX camera). That's probably why the field of view is narrower and the same effective focal length. Since you can't actually adapt (with an adapter) lens from Mirrorlens on DSLR (because of flange distance). And they didn't create a completely new lens. They probably keep the numbers for marketing reasons even though the focal length is probably more likely 16mm (for Full Frame DLSR).

EDIT: just found this comment on dpreview:

Be advised the TTArtisan 11/2.8 fish-eye is actually a 15 mm focal length (actually slightly less, like 14.7 or something). They call it a "11 mm" because when de-fished it provides the FoV of an 11mm rectilinear lens.

What are my options for full frame circular fish eye?