In applications that require large acceptance angles, such as light gathering for illumination, spherical lenses are unsuitable due to spherical aberration, or the effect of refractive power of a spherical surface becoming greater with increasing aperture. Aspheric lenses maintain constant focal length or very high NA, avoiding the need for multiple elements to correct spherical aberration. This simplifies system design by reducing weight and component count. It also results in less transmission loss, and less ghosting due to having fewer surfaces. The primary drawback of aspheric lenses is off-axis performance is poor. This is not a problem for coupling to and from optical fibers or collimating light sources, but aspheric lenses are not usable over a wide field of view.