An ultra-fast standard lens with the largest aperture for SLR lenses in the world at the time. Two large-diameter ground and polished glass aspherical lens elements (3rd & 8th) provide high-contrast with low-flare image quality even at maximum aperture. Curvature of field is minimized through use of four high-refraction glass elements. Spherical aberration and curvature of field at close focusing distance are minimized by floating mechanism. A ring USM (Ultrasonic Motor) achieves silent, high-speed AF and full-time manual focusing.