Nikon's early circular fisheye — the manual-focus 7.5mm f/5.6 for the F mount, 180-degree coverage.
The Fisheye-Nikkor 7.5mm f/5.6 is an ultra-wide circular fisheye for the Nikon F system, introduced in the mid-1960s. It belongs to the early family of Nikkor fisheyes designed to project a circular image well inside the 24x36mm frame, and it required the reflex mirror to be locked up because the rear element sits deep in the camera throat. It captured a 180-degree angle of view for scientific, meteorological and architectural documentation work.
This is a manual-focus Nikon F lens with a maximum aperture of f/5.6 and a fixed 7.5mm focal length. It produces a circular image rather than a full-frame fisheye projection, and it uses a mirror-lock-up mounting scheme with an accessory viewfinder for composition. Filters are handled through a built-in rotating turret rather than a front thread. Exact element counts and weight vary by reference, so those figures are omitted here.
Rendering is defined by extreme barrel distortion that bends all straight lines away from the centre, the signature look of a circular fisheye. The tiny circular frame within the negative suits sky studies, whole-room interiors and full-hemisphere records where geometric accuracy is less important than coverage. It is a specialist optic rather than a general-purpose wide angle.
On the used market these early fisheyes are scarce and command collector prices, so most buyers are Nikon system collectors rather than working photographers. Check the rear group for haze and separation, confirm the mirror-lock-up mechanism engages cleanly, and inspect the built-in filter turret for smooth rotation. Because it demands mirror lock-up, it is not usable on many standard bodies without care, and adapting to mirrorless is impractical due to the deep rear protrusion.