Definitions for "ASPHERIC LENS"
Vision-correction lens material that is thin and light, especially when combined with high index materials. Aspherics have curves that are nonspherical and change gradually from the center to the outside of the lens. Aspherics are now available in single vision, bifocal, and progressive addition designs.
Aspheric means non-spherical but, in optics, it relates to a lens which has one or more of its surfaces not spherical but of a different curvature.  More expensive than spherical lenses but gives fewer errors.
A lens element that changes shape across its surface as opposed to one having a smooth continuous arc. Generally, an aspherical lens deviates slightly from an exactly spherical shape, and is relatively free from aberrations. Light rays are bent more at the edges of a conventional spherical lens than they are at the center, causing them to come into focus before the film plane. A lens made with aspherical elements focuses all the light rays passing through it on the film plane.