Sigma's 1990s constant-aperture tele zoom — 70-210mm f/2.8 APO in Canon EF fit.
The Sigma 70-210mm f/2.8 APO is a constant-aperture telephoto zoom introduced in 1992 during the autofocus SLR era. It gave budget-conscious sport and portrait shooters a fast tele zoom as an alternative to far pricier first-party 80-200mm f/2.8 optics from Canon and Nikon, and Sigma also licensed the design to Vivitar as a Series 1.
The optical construction is 17 elements in 13 groups, using APO low-dispersion glass to suppress chromatic aberration. It weighs roughly 1,300g, takes 77mm filters and focuses to about 1.8m. The diaphragm uses nine blades. An internal-focusing design keeps the front element from rotating, and autofocus is driven by the camera body's screw-drive motor.
Its constant f/2.8 across the range and APO glass separated it from slower consumer zooms of the period, at a fraction of the cost of first-party 80-200mm f/2.8 lenses. The heavy metal barrel and rendering suit portraiture and daylight sport, though it is bulky and slow to autofocus by modern standards.
On the used market condition varies by mount and age; check the APO elements for internal haze and fungus, confirm the screw-drive autofocus engages, and test the zoom and focus action for smooth travel. Adapting to mirrorless is possible but retained autofocus depends on the original fitting.