The Canon EF 20mm f2.8 USM lens is made of 11 elements in 9 groups. It was introduced in 1991 and is the only ultrawide Canon EF lens besides the 14mm f2.8 lens. This lens can be used in all Canon cameras but keep in mind that with APS-C sensor Canon cameras, such as the Canon 7D, this lens is the equivalent of a 32mm lens. Its filter thread is 72mm and it weighs 14.2 oz (405g). One great feature of the Canon EF 20mm f2.8 lens is the IFC, the Intelligent Fielf Curvature, which means that objects away from the center are better focused if they are closer to the camera.