Sigma's constant-aperture ultra-wide zoom for APS-C DSLRs, sharper than the f/4-5.6 version.
The Sigma 10-20mm f/3.5 EX DC HSM is the constant-aperture version of Sigma's popular APS-C ultra-wide zoom, launched in 2009. With a full stop advantage at the wide end over the variable f/4-5.6 version, it provides better low-light capability for landscapes, astrophotography, and architecture.
Optical quality is a clear improvement over the f/4-5.6 version with sharper corners and better controlled distortion. The constant f/3.5 aperture maintains consistent exposure across the zoom range. HSM autofocus is reasonably fast and quiet. Build is solid plastic with a quality feel.
Available in Canon EF-S, Nikon F, Sony A, Pentax K, and Sigma SA mounts. Filter thread is 82mm — larger than the f/4-5.6 version's 77mm. Each mount version is optically identical.
Good value used in all mounts. Check HSM motor and front element condition. The constant f/3.5 is the key advantage over the cheaper variable-aperture version. For mirrorless users the Sigma 10-18mm f/2.8 DC DN is the modern successor.