Konica's rectilinear AR ultra-wide — the 21mm f/4 Hexanon for corrected wide-angle work.
The Konica Hexanon AR 21mm f/4 was an ultra-wide prime for the Konica AR bayonet fitted to Konica's Autoreflex SLR system. It occupied the wide end of the AR line-up as a rectilinear alternative to the fish-eye, aimed at photographers needing a broad, geometrically corrected angle of view.
This is a manual-focus Konica AR lens with a 21mm focal length and a maximum aperture of f/4. It is a rectilinear ultra-wide, so straight lines stay straight rather than bowing as they would through a fish-eye. Optical construction, filter thread and minimum focus are not verified here and are omitted rather than invented.
A 21mm rectilinear wide takes in a sweeping field while keeping architecture and horizons true, which makes it a natural fit for landscapes, interiors and cramped city streets. Stopped down it holds depth of field across most of the frame, so it works well for environmental and travel work where context matters.
Wide Hexanons in the f/4 class turn up less often than the standard primes and tend to be priced accordingly. Inspect the front and rear elements for haze, coating wear and fungus, verify the aperture blades are dry and snappy, and check the focus ring for even resistance. AR-to-mirrorless adapters exist, though the AR flange distance complicates DSLR use.