A shot from earlier today, acquired with the 16/2.8 Fisheye-Nikkor AIS on the D500. The rear-mounted filter has been removed in order to allow some extra cm closer focus, yet the background appear reasonably well defined at the f/16 aperture setting.
Whilst the f/3.5 Fisheye is more than a tad sharper, I selected the f/2.8 because of its ability to focus closer (in particular after Erik worked his magic on it) strengthened by removal of the rear filter. The f/3.5 Fish uses an internal filter wheel thus one cannot play the same tricks on it. Also the f/2.8 is my preferred alternative for IR and IR false-colour shooting.
(16/2.8 Fisheye-Nikkor AIS, ISO 200, f/16 1/100 sec)