Sigma's 70-210mm f/4.5-5.6 was one of several telephoto zooms the company offered through the manual-focus 1980s, giving SLR owners a portrait-to-sport reach without the cost of a camera-maker's own long lens.
The zoom covers medium to long telephoto with a maximum aperture narrowing from f/4.5 to f/5.6, operated with manual focus and a manual zoom ring. Sigma manufactured it for Canon FD, Nikon F, Pentax K, Minolta MD, Olympus OM, M42, Konica AR and Contax/Yashica mounts, with multi-coated glass and an all-metal mount flange typical of the period.
Compared with faster 80-200mm f/3.5-4 designs of the era, this variable-aperture version is lighter and cheaper but needs more light or a higher ISO film to keep shutter speeds usable at the long end. Sharpness is acceptable in the middle of the range and softens toward 210mm.
Used prices are low across every mount. Check for zoom creep when the lens is pointed down, examine the elements for haze that commonly develops in long-stored telephoto zooms, and confirm the aperture stays consistent through the ring's full travel.