Pentax's Tele-Takumar 300mm f/6.3 — a compact slow M42 long-telephoto prime.
The Tele-Takumar 300mm f/6.3 is a long telephoto prime made by Asahi Optical for the M42 screw mount in the early 1960s. Bearing the Tele-Takumar name used for some longer telephotos, it was the reach lens of the early Pentax system for distant subjects, typically a preset-style optic of its generation.
This is a manual-focus M42 screw-mount lens with a 300mm focal length and a modest maximum aperture of f/6.3. Focus is by hand over a long helicoid, and the aperture on early Tele-Takumars is commonly a preset design set on the barrel. It is a long all-metal telephoto. Optical construction details are not asserted here beyond the verified focal length and aperture.
At 300mm the lens reaches distant wildlife, sports and landscape details, and the relatively slow f/6.3 aperture keeps it comparatively compact and light for its length. Long Takumars of this type render cleanly when stopped down and are best used on a support given the focal length. It suits patient long-lens shooting rather than fast action.
Used copies are affordable given the slow aperture and are less common than the standard primes. Inspect the long barrel for internal haze and fungus, confirm the aperture mechanism and any preset ring work smoothly, and check focus over the long travel. A tripod collar or good support helps. An M42 adapter allows manual long-lens use on mirrorless.