Sigma's older 28-70mm f/2.8 constant zoom — pre-EX era DSLR standard zoom.
The Sigma 28-70mm f/2.8D EX Aspherical DF is an older constant f/2.8 standard zoom predating the later EX DG and Art versions. It offered professional-grade constant aperture at a lower price than Canon and Nikon alternatives of the late 1990s.
Optically acceptable — sharp in the centre at mid-apertures. The constant f/2.8 provides consistent exposure. Aspherical elements help control distortion. DF (Dual Focus) refers to a specific focus mechanism design. EX build quality with metal barrel.
Available for multiple mounts. Approximately 82mm filter thread. Full-frame coverage. No stabilisation. D-type communication on Nikon mount. EX professional tier construction. Metal mount and barrel.
Available used very cheaply. Multiple generations of Sigma 24-70mm/28-70mm f/2.8 lenses have superseded this — the Art-line is dramatically better. Only worth considering at bargain prices. Check for smooth zoom and focus ring operation.