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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

TOKINA For wide-angle, our 12-24mm ƒ/4 is best. It's an ƒ/4 aperture rather than an ƒ/2.8, so it's a little bit smaller, it has the same zoom range, and it's an excellent lens. If you want one lens that goes from a wide angle to a standard focal length, our 16-50mm ƒ/2.8 lens, made for a subframe sensor, is great. It's small and lightweight, although any fast lens is going to be heavier. An ƒ/2.8 constant lens is just going to be larger and heavier than an ƒ/3.5-5.6. But if you want that extra image quality, extra build quality and extra speed for the light, you'll just have to make that trade-off. —Michael Burnham

Tamron, according to Costantini, has a line of fast lenses that utilize a special glass called XR, or extra refractive, also used for Nikon, Canon, Sony, Sigma and Pentax lenses. “XR,” he says, “is a highly transmissive type of glass, so by integrating the mechanical technology along with the optical technology, we were able to decrease the size of the lens. The extra-refractive element is highly transmissive, so it basically has a lot of light-countering capability, and it's more efficient than other types of elements. By integrating that into the design with everything else, we were able to make a really small, compact lens.”

Canon's optics are also tailor-made for its line of sub-full-frame cameras, but in the EF-S line of lenses, there's only one lens with a fast ƒ/2.8: the EF-S 17-55mm IS. In its EF line of professional lenses, however, there are a few lenses with a constant aperture of ƒ/4 that work on Canon's full-frame sensors as well as its lighter and more economical APS-sized image sensors.

“The smaller maximum aperture of the ƒ/4 L-series zoom lenses usually makes its possible for us to use fewer total lens elements with smaller maximum diameters,” Westfall says. “This, in turn, helps us to reduce the size, weight and cost of these lenses, compared to the ƒ/2.8 L-series zoom lenses, without a reduction in optical quality or ruggedness.”

The Final Element
Ultimately, for full-frame or sub-full-frame, the speed of the lens is dictated most by how much work and effort is placed into its design and its construction.

“In the development of a lens like the Olympus 300mm ƒ/2.8,” sums up Knaur, “there are really only a few technicians in the world who know how to correctly polish that lens… It's a slow polishing technique—it takes them about sixteen hours to polish one glass element with little or no deviation on the surface. To give an example of the level of quality in the polishing process, if the front element of the 300mm lens were equal in size to Yankee Stadium, any deviation in the surface of the lens would be less than the thickness of one strand of human hair. That's the level of precision that we're talking about.”

OLYMPUS The Olympus 90-250mm ƒ/2.8 is a fast telephoto zoom that offers pros ƒ/2.8 in a reasonably compact package. It's not a small lens, but when you consider that it has a 35mm equivalency of 180-500mm, it's amazingly compact. Action in low light can be shot wide open at a fast shutter speed with a 500mm perspective. ED glass elements eliminate chromatic aberration with the lens wide open, and it's dustproof and splashproof for use in severe conditions. —John Knaur