Constant f/3.5 Sigma ultra-wide for Pentax APS-C with HSM AF, the best-specified ultra-wide zoom for the K-mount system.
The Sigma 10-20mm f/3.5 EX DC HSM is an ultra-wide zoom for Pentax APS-C cameras with a constant f/3.5 aperture — a significant advantage over the variable-aperture f/4-5.6 version. The constant aperture simplifies exposure and provides a brighter viewfinder image across the entire zoom range.
The 10mm ultra-wide provides a 15mm equivalent perspective on APS-C with a dramatic 102-degree angle of view. The constant f/3.5 makes this lens more capable in low light and for astrophotography than the variable-aperture alternative. The HSM motor provides fast, quiet autofocus with manual override.
Build quality is Sigma EX premium tier with quality barrel construction and smooth mechanical operation. The lens is designed specifically for APS-C sensors with the DC designation confirming crop-sensor-only coverage. The standard 82mm filter thread accommodates standard screw-in filters.
The Sigma 10-20mm f/3.5 offers excellent value on the used market as the best-specified ultra-wide zoom for Pentax APS-C. The constant aperture, HSM autofocus, and EX build quality provide a premium shooting experience at prices well below new purchase.