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 a wide-angle zoom for APS-C bodies, providing a 15-30mm full-frame equivalent range with constant f/3.5 aperture — a meaningful advantage over the f/4-5.6 variable aperture of competing consumer ultra-wide zooms. The HSM (Hyper Sonic Motor) provides fast, quiet autofocus. The DC designation confirms APS-C-only image circle — the lens cannot be used on full-frame bodies without vignetting. Available in Canon EF, Nikon F, and Sony/Minolta A mount versions.
The optical design uses 14 elements in 10 groups. Six aperture blades produce out-of-focus rendering at wider apertures. The 82mm filter thread is large for an APS-C lens, requiring a large-format filter set. At 520g the lens is substantial. Minimum focus distance of 0.24m enables wide-angle close focus for dramatic foreground perspective. Constant f/3.5 across the full 10-20mm range. HSM motor provides fast, quiet AF with full-time manual focus override. APS-C-only image circle — not for full-frame use.
The constant f/3.5 aperture distinguishes the 10-20mm f/3.5 from variable-aperture alternatives: for astrophotography, architecture, and low-light indoor wide-angle use where f/3.5 at the longest focal length matters, this lens provides consistent exposure and depth-of-field control across the zoom range. The 15-30mm equivalent on APS-C covers from wide environmental perspective to moderate wide coverage. The 0.24m MFD enables striking wide-angle close-focus compositions with dramatic perspective exaggeration.
On the used market the Sigma 10-20mm f/3.5 EX DC HSM is available at moderate prices for an APS-C ultra-wide. Condition checks: HSM AF response (test for fast, smooth acquisition), 82mm filter thread for any damage, and front element for cleaning marks — the 82mm front makes filters expensive, so front element condition is particularly relevant. The Sigma 10-20mm f/4-5.6 EX DC HSM is available at lower cost. Compatible with APS-C bodies in the specified mount version; using on full-frame bodies is not recommended due to APS-C-only image circle.