Nikon's Ai full-frame fisheye — the manual-focus 16mm f/3.5 for the F mount, 180-degree diagonal view.
The Ai Fisheye-Nikkor 16mm f/3.5 is the Ai-coupled version of Nikon's full-frame fisheye, released in 1977 alongside the wider Ai transition. It carries the mechanical Automatic Indexing ridge for open-aperture metering while keeping the 180-degree diagonal, full-frame projection of the earlier Auto model.
This is a manual-focus Nikon F lens with a maximum aperture of f/3.5 and a fixed 16mm focal length, using the Ai aperture-indexing coupling. It fills the whole 24x36mm frame with a 180-degree diagonal image and carries built-in filters on a rear turret rather than a front thread. Only verified figures are stated; construction details are omitted where sources disagree.
Rendering is the full-frame fisheye look, with barrel distortion bending peripheral lines while the centre stays comparatively natural. It suits creative wide work, action and skate photography, tight interiors, and landscapes where a curved horizon is used for effect. Keeping the horizon centred reduces the bending when a straighter look is wanted.
On the used market this Ai fisheye is a popular choice for photographers wanting the effect with modern metering support, and prices are moderate for a specialist optic. Inspect for front-element scratches, internal haze and fungus, verify the filter turret rotates, and confirm the Ai coupling is intact. It adapts cleanly to mirrorless without mirror-lock-up issues.