The term aspheric has been used as a buzz word quite a bit in the past ten years by optics companies. I thought I'd give some info about them since the optics producers are somewhat secretive about the subject. Any optical shape that is not spherical is considered aspherical, except plano/flat and toric. There are two basis types; Conic Section(also called Cartesian) and complex(also called Schmidt Type). Both types have been used in lenses but by far the most common are the Conic Section shapes (ellipsoid,hyperboloid, paraboloid). Ellipsoids are used as the front surface on a positive lens element with the rear being spherical. These are called sphero-elliptical elements. These can be made into negative elements as well. When the hyperboloid shape is used, it can also be used on negative or positive elements. There have been some double convex hyperboloids made where both surfaces were hyperboloid. The most common configuration for this type of conic surface is plano-hyperboloid. I have never seen paraboloid surfaces used in lenses, only mirrors. Schmidt types have a complex curve that is used in conjunction with elements that have not been corrected for spherical aberration. The Schmidt surface came from the telescope world. In most cases, both the Conic type and Schmidt type of surfaces are used at the rear of the lens to correct out the remaining spherical aberration. Is it essential to use aspherical surfaces to have a high degree of spherical correction? No. Designers have perfected the use of spherical surfaces so well that the difference between lenses with aspherics and not are minimal. SR

-- Steve Rasmussen (srasmuss@flash.net), May 08, 2000