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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

We have a couple of fast zoom lenses. We have the 18-50mm ƒ/2.8 EX DC Macro HSM, which compared to 35mm would be roughly equivalent to 28-80mm. In the telephoto range, we have the APO 50-150mm ƒ/2.8 II EX DC HSM, which is roughly comparable to a 75-220mm zoom on a film camera. This apochromatic lens uses lots of low- dispersion glass, internal focusing and internal zooming, and incorporates the quiet HyperSonic focusing Motor (HSM).
—Tom Sobey

“Critical aperture for any lens is going to be between two and three stops down from wide open,” continues Burnham. “A lens will come out, and it performs great at ƒ/4 to ƒ/16, but that one stop between ƒ/4 and ƒ/2.8 makes the difference. The size of the optics has to be larger, so the expense is higher. They take a look at the expense of creating an ƒ/2.8 lens, and at the optical quality at ƒ/2.8, and say, ‘Let's keep it at a certain price point rather than trying to go to a 2.8.'”

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.
Pentax's top lenses are our DA* line—the 16-50mm ƒ/2.8 lens and the 50-135mm ƒ/2.8 lens. They use our highest quality of glass for maximum resolution, contain our SDM focusing system, which makes focusing virtually silent and extremely accurate, and are weather- sealed to handle even the harshest elements. All offer large maximum apertures relative to their focal length, giving them excellent low-light performance. Plus, since Pentax uses a body-based shake-reduction mechanism, no compromises to lens size or image quality are made in these high-quality, relatively compact lenses.
—John Carlson

While not one of Seidel's original five aberrations, it represents a major breakthrough in optical lens design. Put simply, chromatic aberration is a type of image and color dispersion that occurs when light rays of varying wavelengths pass through optical glass. In the past, correcting this problem for telephoto lenses required special optical elements that offer anomalous dispersion characteristics—specifically, calcium fluoride crystals. However, fluorite easily cracks and is sensitive to temperature changes that can adversely affect focusing by altering the lens' refractive index. So Nikon designers and engineers put their heads together and came up with ED glass, which offers all the benefits yet none of the drawbacks of calcium fluorite-based glass. With this innovation, Nikon developed several types of ED glass suitable for various lenses. They deliver stunning sharpness and contrast even at their largest apertures.”

“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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