Photographic lenses are generally constructed of several single lens elements, all of which, unless otherwise specified, have spherical surfaces. Because all surfaces are spherical, it becomes especially difficult to correct spherical aberration in large-aperture lenses and distortion in super-wide-angle lenses. A special lens element with a surface curved with the ideal shape to correct these aberrations, i.e., a lens having a free-curved surface which is not spherical, is called an aspherical lens. The theory and usefulness of aspherical lenses have been known since the early days of lens making, but due to the extreme difficulty of actually processing and accurately measuring aspherical surfaces, practical aspherical lens manufacturing methods were not realized until fairly recently. The first SLR photographic lens to incorporate an aspherical lens was Canon s FD 55mm f/1.2AL released in March 1971. (Leica offered the 50mm f/1.2 Noctilux lens with aspherical surfaces for its rangefinder cameras many years before 1971.) Due to revolutionary advances in production technology since that time, Canon s current EF lens group makes abundant use of various aspherical lens types such as ground and polished glass aspherical lens elements, ultra-precision glass molded (GMo) aspherical lens elements, composite aspherical lens elements and replica aspherical lens elements.canonglossaryeu
A lens with edges flattened so that it is not a perfect sphere. These produce a much superior image.
A lens with a curved, non-spherical surface. Used to reduce aberrations and enable a more compact lens size. Aspherical lenses minimize coma and other types of lens aberrations, even when used at the widest aperture. They are particularly useful in correcting distortion in wide-angle lenses and help contribute to a lighter, more compact design by reducing the number of standard (spherical) elements necessary. Aspherical lens elements correct these distortions by continuously changing the refractive index from the center of the lens.
A lens element that doesn t form part of a sphere. This is physically difficult to manufacture as most grinding/polishing equipment operates with a spherical action. This sort of lens is often moulded out of plastic or coated onto a glass element.
A lens whose curvedsurface does not conform to the shape of a sphere; lenses are usually ground or moldedwith spherical surfaces; because a spherical surface lens has difficulty in correctingdistortion in ultra-wideangle lenses or coma in large-aperture lenses brought aboutby spherical aberration, an aspherical lens is used.
lens with a curved, nonspherical surface. Used to reduce aberrations and enable a more compact lens size.