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Monday, January 29, 2007

Lens Buying Guide

Everything you need to know about focal lengths, maximum apertures, new technologies and more!

Sometimes subtle tweaks can be important. Portrait photographers, for example, use medium-telephoto lenses—from about 75mm to 105mm—to shoot head-and-shoulders images because a shorter lens—like a 50mm—can distort a person's face when you come close enough to fill the frame.

Lenses For D-SLRs With Smaller Image Sensors
A special class of lenses now offers true wide-angle coverage for D-SLRs whose image sensors have magnification factors of 1.5x or more. These "digital-only" lenses cover just the smaller sensors' image area, allowing designers to create ultrashort focal lengths to compensate for the D-SLRs' magnification factors. They offer the 35mm equivalent of 18mm or wider while maintaining high optical quality. If these lenses have a downside, it's that they won't function with film SLRs, or with D-SLRs using full-frame or nearly full-frame imagers.

Zoom Lenses
As with everything else in life, there are trade-offs to make when selecting lenses. Zoom lenses offer faster access to a variety of focal lengths than fixed-focal-length lenses, and they can be set at in-between lengths like 33mm or 165mm. In some cases, a single zoom can replace a gadget bag full of lenses. Then again, most zooms aren't as fast as their single-focal-length counterparts.

Some zooms offer maximum apertures that remain constant throughout the zoom range. As mentioned, they're generally larger and costlier than zooms with variable maximum apertures. This latter group provides maximum openings that are from about a 1⁄2 stop to 11⁄2 stops faster at a zoom's shortest focal lengths than at the longest ones.

For example, a 28-105mm ƒ/3.5-4.5 zoom has a maximum aperture of ƒ/3.5 at the 28mm setting and ƒ/4.5 at the 105mm end. With telephoto zooms especially, you'll notice the lens speed is reduced right where you need it most—at the long end—but the advantages of compactness and greater affordability may be more important to you.




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