"Sigma 20mm F/1.4 ART DG HSM Lens is a full-frame lens. This revolutionary prime lens is designed to bring out the full potential of ultra-high-megapixel DSLR cameras. With a focal length of 20mm and F1.4 aperture, this lens delivers outstanding large-aperture brightness and bokeh. Through decades of experience in lens design and Sigma Global Vision lens craftsmanship, the 20mm incorporates both ""F"" Low Dispersion (FLD) glass, and five Special Low Dispersion (SLD) glass in a design of 15 elements in 11 groups. The combination of advanced optics and optimized lens power distribution minimizes spherical aberration, axial chromatic aberration and field curvature, producing outstanding image quality. The wide angle and outstanding image quality make this lens great for seascape, cityscape, and astrophotography, as well as for photos in low light, indoor photography, portraits, and event and wedding photography. From the start of the design process, Sigma took steps to minimize flare and ghosting, and established an optical design that is resistant to strong incidental light sources such as backlight with its Super Multi-Layer Coating. As with all Sigma Global Vision lenses, the 20mm F1.4 lens incorporates the latest Sigma technology, including an optimized autofocus (AF) algorithm for smooth, fast, and accurate focusing. In addition, Thermally Stable Composite (TSC) compound material is used, which has a high affinity to metal parts, consistently p|Hyper Sonic Motor (HSM) that ensures a silent, high-speed AF function. Smoother AF is achieved when the this AF algorithm is optimized|Nine-blade, rounded diaphragm: creates an attractive blur to the out-of-focusareas of the image|Brass bayonet mount: of highly precise and durable construction to enhance mount's strength and long-term wear resistance"