 Wayne Turk |
Aspherical Lens ? I have noticed that a number of lens manufactures have some lens that they call Aspherical. What does this mean, and are these lens better then non-aspherical ones ? Thank You
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Pete H |
From "Leica-Culture" "Spherical lenses refract incoming rays more strongly near the edges of an image than they do at the center of an image, thus creating several points of focus, which leads to unsharp pictures. This imaging error is called spherical aberration or aperture error, and it is evident mainly in fast, high-aperture lenses. The best way to correct spherical aberration is the use of aspherical lens elements. Because aspherical lenses have more than one radius of curvature, their fabrication is extremely complex and expensive."
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Will Turner |
"Because aspherical lenses have more than one radius of curvature, their fabrication is extremely complex and expensive." Until the invention of plastic and molded aspherical lens elements, that is. Hybrid aspherical elements are relatively cheap to produce and used on many cheaper consumer-grade zoom lenses. One type uses a glass lens with an additional plastic surface which forms the aspherical shape. The Leica material refers to ground aspherical lenses. The grinding process of a spherical element and the subsequent polishing process is very expensive so you only find them in high-end lenses. In lenses that are significantly affected by spherical aberration, image quality can improve quite a bit. Of course, even in this category there are about a dozen other optical effects that have to be addressed as well, some lenses succeed better than others. The gain of quality by using hybrid aspherical lenses is limited, and often depends on the extent of spherical aberration generated by the lens design itself - fast zooms seem to suffer the most. One thing's for sure, simply adding an aspherical element doesn't make it a Leitz.
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Michael H. Cothran |
Wayne - Here's what it is - A "normal" lens surface is round, curved, or SPHERICAL in design. That is, symmetrical in its 'roundness' from one side to the other. An "A-SPHERICAL" lens is shaped differently. It is ASYMMETRICAL in shape, maybe rounder in the middle than on the edges, maybe with a soft plateau halfway in. These asymmetrical designs allow the lens element to bend light at different angles at the edges than in the center. These ashperical lens elements used to be expensive to make, but with the technology over the past decade or so, many manufacturers are designing aspherical elements using plastic lenses rather than glass. This is why some lenses who advertise having aspherical lens elements within are surprisingly inexpensive. In the world of optics, you still get what you pay for. There's a reason why Leitz, Zeiss, and Schneider lenses are so expensive. Don't think for a minute that your $99 third world lens with the aspheric element and low dispersion glass will make up for it. Cheaply made Aspheric lens elements and/or Low Dispursion glass in a cheaply made lens will not transform it into a world performer. It will only transform the cheap lens into a slightly better performer than what it would be without aspherical elements and/or low dispersion glass. Which may not be saying much! Michael H. Cothran
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