Sigma 135mm f/3.5 — compact manual-focus telephoto prime sold across nearly every 1970s SLR mount.
The Sigma 135mm f/3.5 was a staple of the company's manual-focus prime lineup through the 1970s and into the 1980s, a compact budget telephoto aimed at SLR owners wanting reach beyond a standard 50mm without spending on a first-party lens.
A simple manual-focus design with a manual aperture ring, the 135mm f/3.5 was manufactured in a wide spread of fittings, among them M42 screw mount, Pentax K, Canon FD, Nikon F, Minolta MD/SR, Olympus OM, Konica AR and Contax/Yashica mounts, reflecting how many SLR systems it was sold alongside.
Optically modest by later standards, it renders with reasonable contrast stopped down to f/5.6-f/8 and a plain, slightly busy out-of-focus background typical of older uncoated-feeling tele designs, though usable for portraits at a distance.
These primes are common and inexpensive secondhand. Check the aperture blades move cleanly without oil, the manual focus ring for grease drag, and confirm the specific mount version matches the intended camera before buying.