Aspherical lenses What is it and what are the available options.
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).
Simplest way to illustrate what a aspherical lens element is all about.
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.
Simplest way to illustrate what a aspherical lens element is all Spherical abberation of a spherical lens and convergence of parallel light rays with the use of a aspherical lens element.about.
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.
The OP-Fisheye Nikkor 10mm f5.6 is one of the earliest in Nikkor family to use an aspherical lens front element to ensure its mathematically correct illumination pattern.
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.
Tamron's Aspherical lens 28-200mm f3.8/5.6 LD IF is only 3.2" in length. First started the 28-200 compact zoom revolution in 1992, current version packs all modern features like low dispersion glass, internal focus and aspherical lens surface.
Another 3.2" compact offer from Tamron, the SP AF20-40mm F/2.7-3.5 Aspherical (IF). Packing features like dual aspherical elements: Large diameter Hybrid Aspheric elements are used in the front focus group to substantially compensate for distortion aberrations at wide angle. Another Hybrid Aspheric element added to the rear group effectively eliminates comatic flare.
Though early days, the concept of adoptions of aspherical design were to certain specialised lenses where its benefits can be fully realised in specific applications. But since with the wide acceptance and popularity of zoom lenses by many serious SLR users nowadays, you will be rest assured that new generation of aspherical zoom and prime lenses will be coming in with more "varieties" and economically - in a grand scale.
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.
First affordable compact Nikkor zoom to utilise an aspherical element within. The AF Zoom Nikkor 28-70 f3.5/4.5 D, defininately will not be the last from Nikon.