Sigma constant f/3.5 ultra-wide for Canon APS-C with HSM, brighter than variable alternative at mid-range pricing.
The Sigma 10-20mm f/3.5 EX DC HSM for Canon EF is the constant-aperture variant of Sigma's ultra-wide APS-C zoom, maintaining f/3.5 throughout the range. The constant aperture provides more consistent brightness and depth of field than the variable f/4-5.6 alternative.
The constant f/3.5 is brighter than the variable version at all focal lengths beyond 10mm, providing a practical advantage for low-light wide-angle and astrophotography. The HSM motor provides fast, quiet autofocus with manual override.
Build quality is Sigma EX premium tier with quality construction. The constant aperture simplifies exposure across the ultra-wide range, and the 10mm provides 16mm equivalent dramatic perspectives on APS-C.
The Sigma 10-20mm f/3.5 constant for Canon offers the brighter ultra-wide option at moderate used pricing. The constant aperture provides genuine practical advantages over the variable version for serious wide-angle work.