Nikon's Ai-coupled circular fisheye — the manual-focus 8mm f/2.8 for the F mount, 180-degree view.
The Ai Fisheye-Nikkor 8mm f/2.8 is the Ai-coupled version of Nikon's circular fisheye, released in 1977 when the whole Nikkor line adopted the Automatic Indexing metering system. It carries the mechanical Ai ridge for open-aperture metering on the bodies of that era while retaining the 180-degree circular projection of the earlier Auto model.
This is a manual-focus Nikon F lens with a maximum aperture of f/2.8 and a fixed 8mm focal length, using the Ai aperture-indexing coupling. It produces a 180-degree circular image within the frame and relies on a built-in filter turret rather than a front thread. Only verified figures are given here; internal construction details vary by source and are omitted.
Rendering is the round fisheye look, with heavy barrel distortion and a black surround around the circular image. It suits sky and cloud studies, whole-room interiors and scientific documentation where a full hemisphere must be recorded in one frame. The vast depth of field means critical focusing is seldom needed.
On the used market this Ai fisheye is scarce and collectible, valued both for its coverage and its Ai metering compatibility. Inspect the front and rear groups for haze, fungus and separation, verify the internal filter turret rotates smoothly, and confirm the Ai coupling ridge is intact for metering bodies. It adapts to mirrorless without mirror-lock-up concerns, though the circular image remains a specialist effect.