Pentax's wide K-mount prime — a manual-focus 18mm f/3.5 with Super Multi Coating.
The SMC Pentax 18mm f/3.5 is a wide-angle rectilinear prime for the Pentax K bayonet mount, from the first-generation K-series range that followed the M42 Takumars. It filled the very-wide slot in the line below the more common 24mm and 28mm focal lengths.
This is a manual-focus lens with an 18mm focal length and a maximum aperture of f/3.5, finished with Pentax's Super Multi Coating to manage flare. Details such as element count, weight and filter thread should be checked against the individual lens rather than assumed, as early wide primes varied in construction.
At 18mm the field of view is wide with a rectilinear projection, keeping architectural lines straight and delivering deep depth of field for landscape and interior work. It gives an expansive look without the curved distortion of a fisheye.
Wide Pentax primes of this vintage appear on the used market less frequently than standard lenses, and clean examples command a premium. Look through the glass for haze and fungus, test the aperture blades for oil, and check the focus and aperture rings for smooth action. It fits Pentax K digital cameras and adapts to mirrorless bodies.