Sigma's Zoom 100-200mm f/4.5 dates from the company's early manual-focus catalog, a narrow-range telephoto zoom sold when fixed-aperture, single-purpose lenses were the norm before variable-aperture designs became widespread.
The lens holds a constant f/4.5 maximum aperture across its 100-200mm range, manually focused and zoomed, and was manufactured for Canon FD, Nikon F, Pentax K, Minolta MD, Olympus OM, M42, Konica AR and Contax/Yashica mount SLRs with multi-coated glass and an all-metal construction typical of Sigma's earlier lens generation.
The constant aperture gives an even viewfinder brightness and predictable exposure through the zoom range, an advantage over later variable-aperture 100-200mm competitors. Build quality leans heavier and more metal-bodied than Sigma's later plastic zooms.
Now a genuinely old design, so condition checks matter: look closely for fungus and haze from decades of storage, test the focus helicoid for smooth even travel, and confirm the aperture blades close down cleanly without sticking.