Sigma's 75-300mm f/4-5.6 was a later, faster-starting successor to the company's earlier 75-300mm designs, sold through the autofocus era as an affordable long-telephoto option for enthusiasts wanting reach without a large budget.
Covering 75-300mm with a maximum aperture running from f/4 at 75mm to f/5.6 at 300mm, the lens autofocuses and was produced for Canon EF, Nikon F, Minolta A, Pentax K and Sigma SA mount bodies, in a lightweight plastic barrel with an extending front zoom group.
The faster f/4 wide-end aperture over older f/4.5-5.6 versions gives a brighter finder and slightly quicker autofocus at 75mm, though performance at the 300mm end still softens and a monopod or fast shutter speed helps counter camera shake.
Widely available and cheap on the used market across most mounts. Test autofocus speed and accuracy at 300mm, check the extending barrel for looseness, and inspect for haze or fungus, both common in long-stored telephoto zooms of this age.