Nikon's slim pancake prime — the manual-focus GN Auto Nikkor 45mm f/2.8 for the F mount, Tessar-type.
The GN Auto Nikkor 45mm f/2.8 is a compact pancake-style prime for the Nikon F system, introduced in the late 1960s. GN stands for Guide Number, a feature that coupled the focus ring to the aperture so that flash exposures could be set automatically by turning focus. Its slim body made it one of the smallest Nikkors of its day.
This is a manual-focus Nikon F lens with a maximum aperture of f/2.8 and a slightly-wide-of-normal 45mm focal length, with an automatic diaphragm and the distinctive GN flash-coupling mechanism. It uses a Tessar-type layout that keeps the lens very thin. Only the verified focal, aperture and mount are stated; other specifics are omitted to avoid error.
The Tessar-type design gives crisp central rendering and a compact, flat-fielded look that many find pleasing for street and travel. Its slim profile makes a Nikon F body noticeably more pocketable, and the 45mm view sits usefully between 35mm and 50mm. The GN coupling is a period curiosity most users now ignore.
On the used market the GN 45mm is a collectible and fairly common compact Nikkor, popular for its size. Inspect the elements for haze and fungus, check coatings for marks, confirm the aperture blades are dry, and test the focus and GN coupling for smooth action. It adapts well to mirrorless where its slim body suits small bodies, and later Nikon revived the 45mm pancake concept.