This Uber-Rare Nikon 6mm f2.8 Fisheye Lens Can See Behind Itself - The Phoblographer
Sample Footage Shot with the Ultra Rare Nikkor 6mm f/2.8 that Can See Behind Itself | PetaPixel
Lensfinder.com - Ultra Rare Nikkor 6mm f/5.6. Only a few hundred of these lenses exist. This lens has a 220 degree field of view for Full Frame 24x36mm sensors. Made in Japan