An attempt by the independent company SIGMA to compete with the major manufacturers in the niche of APS-C digital SLR cameras. All cameras of this system, belonging to the SD family, were based on the unique Foveon X3 technology, the world's first image sensor that featured three layers of photo detectors - one layer recorded red, another layer recorded green and the other layer recorded blue light - at each and every pixel. That's why the Foveon X3 technology delivered twice the sharpness, better color detail and greater resistance to unpredictable color artifacts. All other image sensors feature just one layer of photo detectors, with just one photo detector per pixel. As a result, these sensors capture only 25% of the red and blue light, and just 50% of the green, and must rely on complex processing to interpolate the colors they miss. Interpolation leads to color artifacts and a loss of image detail.
SIGMA did not produce SA-mount lenses exclusively for their SD series SLRs: these were always models intended for cameras from major manufacturers PLUS cameras of the Sigma SA APS-C system. The lineup was not particularly extensive, but included models with practical focal lengths and speeds: fisheyes (4.5mm, 10mm), 30mm F/1.4 standard lenses, wide-angle zooms (8-16mm, 10-20mm), standard zooms (17-50mm, 17-70mm, 18-50mm), telephoto zooms (50-150mm, 50/55-200mm), superzooms (18-125/200/250/300mm), and even models with unique specifications (18-35mm F/1.8, 50-100mm F/1.8).
Development of new cameras and lenses with the Sigma SA mount was discontinued in September 2018.