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Tuesday, March 1, 2005

Gadget Bag: New Lenses In A Digital Age

The latest designs bring true wide-angle capability and more to the digital format

You've no doubt seen or heard about new "designed for digital" lenses that are optimized for better performance with digital image sensors. If you're like most people, though, you're not quite sure what that means—just what makes a lens that's "designed for digital"? We'll cover the details and show you how the new lenses have helped overcome some of the challenges once posed by a switch to digital, and how these advanced optics can improve your images.

new lensesD-SLR Magnification Factors
One of the most important differences between film and digital SLRs is the smaller size of most D-SLRs' image sensors. With any lens you use, the angle of view seen by a D-SLR's sensor is less than that seen across the full span of the 35mm frame.

This narrower angle of view is equally well-described as greater magnification, and the image recorded with your lens on a D-SLR appears similar to one taken with a much longer lens on a 35mm camera. This effect gives us the magnification factor familiar to D-SLR shooters. (If you've ever shot 4x5 film, you've experienced something similar—a 90mm lens is a wide-angle with the big film format and a telephoto for the smaller 35mm frame.)

To gauge how your 35mm lenses will behave on a D-SLR, multiply the focal length by the camera's magnification factor. For example, a 200mm lens on a D-SLR with a 1.5x factor gives the equivalent of a 300mm focal length. It sure looks like a case of getting something for nothing—telephoto users certainly can get the extra reach of the longer lens without the increased weight or expense. The problem is that wide-angles' equivalent focal lengths are increased by the same amount.

On a film camera, a 20mm lens is nearly an ultra-wide. With a D-SLR's 1.5x magnification factor, though, it takes in no broader an angle than a moderately wide 30mm lens would on a film SLR. To get the same angle of view with a D-SLR as you'd get shooting a 20mm on a film camera, you'd need an ultra-short 12mm or 13mm lens, and these optics simply didn't exist much more than a year ago.

Wide Lenses For D-SLRs

Lens makers have given us back our wide-angle coverage with special optics designed specifically for the small-format image sensors found in most D-SLRs. Because these lenses don't have to cover the larger image area of a 35mm camera or a full-frame (or nearly full-frame) D-SLR, optical designers can create more radical lenses, including focal lengths of 12mm or even shorter. Mounted on a D-SLR, the lenses perform much like the wide-angle lenses we depend on with 35mm cameras.

Because of the recent advent of these designs, the lenses can capitalize on the latest in lens-making technology, including widespread use of low-dispersion (LD) glass and aspherical lens elements. LD glass minimizes the color fringing associated with chromatic aberrations in telephoto lenses. Because the ultra-short wide-angles use an inverted-telephoto design, LD glass is important for them, too. Aspheric lens elements minimize distortion and help improve sharpness in these technically demanding optics.

Examples Of Modern Digital Lenses
The Nikon DX series set the stage with lenses like the 12-24mm ƒ/4 G ED-IF AF-S DX Zoom-Nikkor. With the 1.5x magnification factor on Nikon D-SLRs, the 12-24mm lens provides a 35mm-equivalent focal length range of 18-36mm. The growing series of lenses includes a 10.5mm fish-eye, and also fits Fujifilm's FinePix S2 Pro and S3 Pro D-SLRs.

The Canon EF-S 10-22mm ƒ/3.5-4.5 USM is designed for the smaller imager in the EOS Digital Rebel and EOS 20D D-SLRs. With the cameras' 1.6x magnification factor, the lens offers the same angle of view as a 16-35mm lens on a 35mm camera, the widest non-fish-eye available on a D-SLR—for now.

Olympus uses the smaller 4/3 format for its D-SLRs' image sensors. Because of the sensors' size, the cameras have a 2x magnification factor, giving the Olympus 11-22mm ƒ/2.8-3.5 Wide Zoom an angle of view equivalent to a 22-44mm lens for the 35mm format. The zoom's two aspherical lens elements promote image quality.


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