Cosina — Pentax K-mount 24mm f/2.8 wide-angle prime from the 1981 Cosinon-W series
The Cosinon-W 24mm f/2.8 extended Cosina's early-1980s wide-angle range beyond its 28mm and 35mm Cosinon-W primes to an ultra-wide focal length, giving K-mount shooters a budget alternative to Pentax's own SMC Pentax-M 24mm f/2.8.
Cosina's specification lists a seven-element, seven-group construction, a 49mm filter thread, five aperture blades, a weight of around 165g, and a 0.25m minimum focus distance. The lens mounts natively in Pentax K bayonet fitting with an f/2.8-f/22 aperture range.
Unlike its later stablemate, the 24mm f/2.8 MC Macro, this Cosinon-W version has a conventional single focus throw with no dedicated close-focusing range, built for general wide-angle work such as interiors and landscapes rather than close-up subjects.
This is the less commonly seen of Cosina's two vintage 24mm f/2.8 designs, since 24mm primes generally sold in smaller numbers than 28mm lenses. Listings should be checked to confirm the lens is genuinely this plain Cosinon-W rather than the visually similar Macro version.