Nikon's original symmetrical super-wide — the manual-focus Nikkor-O Auto 21mm f/4, mirror-lock-up design.
The Nikkor-O Auto 21mm f/4 is one of the original lenses launched with the Nikon F in 1959, an ultra-wide of symmetrical design. The O code denotes its element count, and unlike later retrofocus wides its rear group protruded deep into the body, so it required the reflex mirror to be locked up during use. It was a specialist super-wide for the earliest F system.
This is a manual-focus Nikon F lens with a maximum aperture of f/4 and a fixed 21mm focal length, built on a near-symmetrical optical layout that needed mirror lock-up and an accessory optical finder. Because the rear element sits close to the film plane it does not permit normal reflex viewing. Only the verified focal, aperture and mount are given; further specifics are omitted.
The symmetrical design gives well-controlled distortion and even field rendering typical of this construction, prized for architectural straight-line fidelity. It suits landscape and architecture on a tripod where the mirror-lock-up workflow is acceptable. The deep rear element is the defining practical trait of the lens.
On the used market this first-generation 21mm is a collector's item, valued as an original Nikon F system lens despite its mirror-lock-up limitation. Inspect the rear group for haze and separation, confirm the aperture blades are clean, and check the mount and finder-coupling for wear. Because of the deep rear element it will not mount on many bodies without care, and mirrorless adaptation needs a compatibility check.