Pentax's SMC Takumar 15mm f/3.5 — a rare multi-coated M42 ultra-wide rectilinear prime.
The Super-Multi-Coated Takumar 15mm f/3.5 is an ultra-wide-angle prime made by Asahi Optical for the M42 screw mount in the early 1970s. It was one of the widest rectilinear lenses in the Takumar range and carried Asahi's Super-Multi-Coating, which improved flare control and contrast. It sat at the specialist wide end of the Pentax M42 system.
This is a manual-focus M42 screw-mount lens with a 15mm focal length and a maximum aperture of f/3.5. Focus and the automatic diaphragm are set on the barrel. As an ultra-wide it is a large, complex optic that in this era typically used built-in filters rather than a front thread. Element counts are not asserted here beyond the verified focal length and aperture.
At 15mm this is a dramatic ultra-wide that renders sweeping perspectives for architecture, interiors and expansive landscapes while remaining rectilinear rather than fisheye. Multi-coating gives it better flare resistance than earlier Takumars. It is a specialist tool valued for the reach of its angle of view and its solid rendering when stopped down.
This is one of the rarer and more valuable Takumars and clean copies are expensive. Inspect the large front element for scratches, haze and fungus, verify the automatic diaphragm and any built-in filter mechanism work, and check focus smoothness. Because of its bulk confirm the mount and barrel are undamaged. An M42 adapter allows use on mirrorless for ultra-wide work.