Sigma's older APS-C standard zoom — f/2.8-4.5 with OS stabilisation, pre-Art era.
The Sigma 18-50mm f/2.8-4.5 DC OS HSM is an older APS-C standard zoom with OS stabilisation, predating the popular 17-50mm f/2.8 EX DC OS HSM. The variable f/2.8-4.5 aperture makes it less desirable than the constant f/2.8 alternatives.
Optically adequate — sharp in the centre at mid-apertures. The OS stabilisation provides approximately 4 stops — a useful feature. HSM motor for quiet AF. The variable aperture starting at f/2.8 wide narrows to f/4.5 at 50mm — less consistent than constant f/2.8.
Available for multiple APS-C DSLR mounts. 67mm filter thread. Approximately 395g — lightweight. No weather sealing. DC designation — APS-C only. Minimum focus distance 30cm.
Available used very cheaply. The Sigma 17-50mm f/2.8 OS is a much better choice with constant aperture. This is an earlier, less desirable lens. Only worth considering at very low prices. Check for smooth OS operation.