The Sigma 12-24mm f/4.5-5.6 EX DG HSM is a professional-level wide-angle zoom lens made by Sigma Corporation. It was once the widest rectilinear lens available for full-frame 35mm SLR cameras, providing a field of view of 122 degrees. Other lenses available with focal lengths of 12mm or less are either incompatible with full-frame cameras or are intentionally distorted. The Sigma 12-24 has low distortion even compared to less wide zooms like the Canon EF 16-35mm lens.
This lens is available in Canon, Nikon, and Sigma, Pentax, and Sony mounts. HSM focusing is available only in the Canon, Nikon, and Sigma variants. The build quality is typical of Sigma EX lenses, with a painted metal lens barrel and damped focus and zoom rings. As expected for a lens of these specifications, there is a large protruding front element.