Aspheric lenses are any lenses whose surface angles and profiles are neither part of a sphere nor part of a cylinder. Conventional lenses have the same curve across their entire surface, like a ping-pong ball. Aspheric lenses accomplish the same amount of refraction but are flatter and slimmer.
These lenses are commonly used in photography, as well as in eyeglasses and contact lenses. In the context of photography, the complex surface profiles of the lenses can eliminate the distortion of certain types of lines, when compared to a conventional lens. A single aspheric lens sometimes has the ability to take the place of a more complex system of multiple lenses. This allows lens systems to be smaller and lighter, in addition to sometimes costing less.
One of the most common uses of aspheric lenses is in vision correction. When used in eyeglasses, they are much thinner and lighter than traditional lenses. This gives a more pleasing aesthetic appearance, which is a matter of at least some concern to most eyeglass wearers. These lenses do not provide conclusively better performance than traditional lenses, but nor is there any compromise in quality of vision. Unlike photographic lens systems, aspheric eyeglass lenses are more expensive to produce than conventional types.
The comparatively flat profile of aspheric lenses is especially noticeable when the lenses correct for severe hyperopia, or farsightedness. Ordinary lenses are much thicker and bulge out in the middle, sometimes causing an awkward magnification of the eye for those with hyperopia. These problems are greatly reduced, however, by the use of aspheric lenses. One of the few disadvantages of this type of eyeglasses is that the lenses tend to be positioned closer to the face, which can bring about visible reflections on the front and back of the lenses. The application of an anti-reflective coating to these lenses is therefore recommended by many eye care professionals.
It is possible and very common for aspheric lenses to be manufactured by molding, and these are very inexpensive and perform reasonably well. They are commonly made for point-and-shoot cameras, and are also part of the cameras that often come integrated into cellular phones. The laser which reads data in a compact disc player is also refracted through a molded aspheric lens. Molded lenses are all small, whereas larger ones must be ground and polished. There are many methods for doing this, with varying degrees of precision, depending on what the lens will be used for.
