Nikon's rare Ai-S rectilinear ultra-wide — the manual-focus 13mm f/5.6 for the F mount, corrected geometry.
The Ai-S Nikkor 13mm f/5.6 is the Ai-S revision of Nikon's ultra-rare rectilinear ultra-wide, updated in the early 1980s to the final manual-focus standard. It kept the extreme corrected wide angle of the original while adding full Ai-S metering compatibility, remaining a special-order optic made in tiny quantities.
This is a manual-focus Nikon F lens with a maximum aperture of f/5.6 and a fixed 13mm focal length, built to the Ai-S standard and designed for rectilinear rather than fisheye rendering. It carries built-in rear filters rather than a front thread owing to its large front element. Only verified figures are stated here; internal construction details are omitted to avoid guessing.
The character is defined by holding straight lines straight across a very wide field, controlling the distortion that would otherwise bow architectural edges. It suits interiors, cityscapes and technical work where geometry must be preserved. The f/5.6 maximum aperture reflects the complexity of the correction rather than a limitation of purpose.
On the used market this Ai-S 13mm is one of the most valuable and rarely traded manual Nikkors, of interest almost entirely to collectors. Check the front element for scratches and coating condition, verify the built-in filter turret rotates, and confirm smooth Ai-S aperture and focus action. Its extreme scarcity and price place it firmly in the collector category.