Nikon's early retrofocus wide — the manual-focus Nikkor-UD Auto 20mm f/3.5 for the F mount.
The Nikkor-UD Auto 20mm f/3.5 is a wide-angle prime for the Nikon F system, introduced in the late 1960s. The UD optical code marks its element grouping, and it was one of Nikon's early retrofocus 20mm designs that let a very wide lens clear the SLR mirror. It served architecture, interior and landscape photographers wanting a dramatic but rectilinear field of view.
This is a manual-focus Nikon F lens with a maximum aperture of f/3.5 and a fixed 20mm focal length, using an automatic diaphragm for full-aperture viewing. It is a rectilinear retrofocus design that clears the reflex mirror for normal operation. Only the verified focal, aperture and mount are stated; element counts, filter thread and weight are omitted to avoid error.
The rendering combines a wide, dramatic field of view with controlled distortion typical of Nikon's early super-wides, giving strong perspective when shooting close to foreground subjects. It suits landscapes, tight interiors and environmental street work. Its moderate f/3.5 aperture is workable handheld in good light, and depth of field is generous.
On the used market the UD 20mm is a collectible early Nikkor wide, valued by system collectors and users of vintage glass. Check the front and rear elements for haze, fungus and coating scratches, confirm the automatic diaphragm snaps closed, and test focus for smooth, even damping. It adapts to mirrorless and remains a usable if characterful wide-angle.