Aspherical lenses |
Aspherical lenses |
What is it ?
Aspherical lenses incorporating some optical characteristics. These lenses are small, lighter and in general, better than similar lenses which only employ spherical elements. In some instances, using an aspherical element enable the lens designer to use fewer lens elements. The benefit can be a smaller, compact and lighter lens; with fewer lens surfaces, there is also less potential for internal reflection - that is why there are more and more zoom lenses incorporating a (or few) aspherical lens element within to subsitute a number (Depends on optical design).
Technically, an aspherical lens is a lens whose curved surface does not conform to the shape of a sphere; lenses are usually ground or molded with spherical surfaces; because a spherical surface lens has difficulty in correcting distortion in ultra-wideangle lenses or coma in large-aperture lenses brought about by spherical aberration, an aspherical lens is used. Where in the case of zoom lenses, a lens element or two can substitute a bigger number of lens elements to achieve similar or better optical results and to reduce the overall cost of production and more compact of the size of a lens - especially in the case of zoom lenses, where lens elements of more than 10 are not too unusual.
Generally, for the vast majority of even the most demanding applications, conventional lenses are much more than adequate. Where previously, due to the high cost and tedious production process of producing aspherical lens elements, most manufacturers who has the technology to produce these elements only restrict the use of aspherical surfaces to lenses where they are essential.
Anyway, the existence of aspherical lens has been quite a while on the market. Like the time honoured Noct-Nikkor 58mm f1.2 and the discontinued OP-Fisheye Nikkor 10mm f5.6 from Nikon has been around for almost two decades are just two classical examples. Each representing lenses for some specific applications, like general or scientific/Industrial use.
Recent development of seeing more and more exciting lenses made possible after individual manufacturer found various solutions in manufacturing process to produce aspherical lens elements. Tamron developed a method of apply and treating an optical resin to normal spherical surface of a lens to made it aspherical; Nikon overcame its problem in grinding etc..all these accounted for the sudden emergence of lenses of all types with a or a few aspherical lens elements within.
Among the top five camera manufacturers in Japan, Canon is the most aggressive among the few to push the technology into their EF-L series lenses. Although many may not realise, the world's first SLR lens to have an aspherical element incorporated within was a FD 55mm f1.2 AL SSC.
Fast lenses, at times has problem maintaining its optical performance at widest opening, especially in situation where the illumination is dim. A aspherical lens can assure optimum correction of coma. Like images of scenes with small points of light - bright point sources of light near the edges of the picture appear as dots rather than comet-shaped blurs. Some newer generation of fast lens, like the world's fastest 28mm lens, the AF Nikkor 28mm f/1.4D, or the recent Canon's super fast EF 50mm f1.0 L use an aspherical element to ensure compact size and to obtain superb performance by eliminating sagittal, or arrow shaped, coma, even at its widest aperture.
Sormetimes, the light passing through the central area of a lens cannot be focused at precisely the same plane as the light passing through the peripheral areas. This is a spherical abberration that makes sharpness inconsistent. While this phenomenon is not always a problem, some lens designs can be significantly improved to higher levels of overall sharpness through the use of aspherical elements.
The solid AF Nikkor 28mm f1.4D is not cheap, one of the first few new generation of AF- Nikkor from Nikon came with an aspherical lens element.
What is it and
what are the available options.
