Monday, April 28, 2008
Fast Lenses On Sub-Full-Frame Sensors
Quick, inexpensive and light, sub-full-frame lenses aren't just for the amateur market anymore
Probably the most important development in lens construction is an aspherical lens design. Applied to all modern digital lenses, aspherical optics give the lens the ability to converge light rays to the same exact focal point from many directions, eliminating aberrations and allowing for fewer elements to be utilized in the construction of a lens.
“Aspherical lenses virtually eliminate the problem of coma and other types of lens aberration, even when used at the widest aperture,” says Silverman. “They are particularly useful in correcting the distortion in wide-angle lenses. In addition, use of aspherical lenses contributes to a lighter and smaller lens design.”
Silverman goes on to say, “Nikon strives for optical design that does full justice to the characteristics of the image sensor. And we've evolved the Nikon standard of optical performance beautifully to work with digital cameras. Nikon developed ED (Extra-low Dispersion) glass to enable the production of lenses that offer superior sharpness and color correction by minimizing chromatic aberration.
“To build a fast lens,” says Olympus' Knaur, “we have to take advantage of the rear mount opening, the ED glass, the super ED glass, aspheric elements, the optical design, the internal focusing included in that design, as well as the overall design of the lens. By using the glass in a certain way, you can create sharpness and control of the light that will allow you to have those fast openings. Aspherics usually show up more so in the wide-angles, so that you maintain the sharpness in the edges.”
All manufacturers use special coatings and low-dispersion glass elements to prevent ghosting and other chromatic aberrations, and also keep the bulkiness and weight down when compared to traditional film lenses, which often are designed for full-frame sensors, anyway. Most of these coatings are proprietary, and contain secret recipes that are as guarded as much as the formula for Coca-Cola.
Pentax, which utilizes APS-C-sized image sensors for its D-SLRs, makes its lineup of DA* lenses with aspherical and extra-low-dispersion glass elements.
“All of our current lenses, including the DA* zoom lenses, are specifically designed for APS-C-sized digital sensors,” Carlson says. “This affects the overall size of the lens and how the lens elements are arranged so that the image circle is appropriate for those sensors. Additionally, there are significant differences between the flatness of digital sensors and film. Film was not perfectly flat, so compromises could be made to lens design. Because digital sensors are perfectly flat, the lenses for digital cameras require a higher level of precision so that spherical aberrations are minimized.”
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