Sigma's older super-telephoto zoom — 170-500mm at f/5-6.3 before the 150-600mm replacement.
The Sigma 170-500mm f/5-6.3 APO DG is an earlier super-telephoto zoom predating the 150-600mm Contemporary and Sports. It provided 170-500mm reach with APO glass — one of the longest zooms available in the early 2000s.
Optically acceptable — sharp at mid-focal lengths with APO glass controlling chromatic aberration. Soft at 500mm. No stabilisation — a significant limitation at these focal lengths. Autofocus adequate. Full-frame coverage.
Available for multiple DSLR mounts. Large filter thread. Very heavy — approximately 1600g. No weather sealing. APO/DG designation. Tripod collar. No OS stabilisation.
Available used very cheaply. The 150-600mm Contemporary and Sports are dramatically better in every respect. No stabilisation makes 500mm extremely challenging handheld. Only for absolute budget super-telephoto use with tripod support.