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Saturday, July 1, 2006

Telephoto Zooms

The following are key considerations when choosing a telephoto zoom lens.

One Ring Or Two?

Some zoom lenses have two control rings—one for focusing and one for zooming. Others use a single ring: rotating it focuses the lens and pushing/pulling it zooms the lens. With the one-ring method, you won't accidentally zoom when you intend to focus, but many photographers are more comfortable with the twin-ring system. Before you buy a tele-zoom, try out the focusing and zoom controls to see how you like the configuration.

Class Glass

If you shoot a lot at wide apertures, you'll probably want a lens that corrects the aberrations that are especially evident at wide apertures. Low-dispersion elements, with such designations as ED, LD, UD, ELD, SLD and SED, correct chromatic aberrations to improve sharpness and contrast as do fluorite elements. Aspherical elements correct spherical aberrations, improving sharpness. The Canon EF 70-300mm ƒ/4-5.6 DO IS USM zoom features multilayer diffractive optics (the "DO" in the name), which cancel out chromatic aberrations produced by conventional optics and correct spherical aberrations as well, improving image quality and allowing for much more compact lenses.

Focusing Considerations

Many lenses employ front focusing, where the front element rotates and moves away from the camera body as you focus from infinity toward the minimum focusing distance. This makes using orientation-sensitive lens attachments such as polarizers and graduated filters difficult. Lenses that employ internal or rear focusing don't rotate the front element, so filters will retain their orientation during focusing. And since these lenses' physical length doesn't change during focusing, there's also less weight shift. Another advantage is that moving only small internal elements instead of the large front ones makes for quicker and more accurate autofocusing.

Some AF lenses permit you to focus manually without leaving AF mode (the Canon USM lenses that have focusing scales and the Nikon AF-S Silent Wave lenses, for example); others provide quick switching from autofocus to manual focus mode (Pentax's Quick- Shift Focus and Tamron's MF/AF Switchover, for example). This can be a real time- and shot-saver.

Many long lenses provide a focusing range limiter. You can set this for full travel (allowing the lens to focus from infinity to its closest focusing distance) or for limited travel (allowing the lens to focus from infinity to about twice its minimum focusing distance or from twice its minimum focusing distance to its minimum focusing distance). By using the range limiter, you'll get much quicker autofocusing performance because the lens won't have to "hunt" through such a great distance range.

"Macro" Capability

Many zoom lenses have the word "macro" in their name. Don't confuse these with true macro lenses, which focus down to 1:1 (life-size) magnification. Macro zooms will focus closer than standard zooms of equivalent focal-length range, but often no closer than a 1:4 reproduction ratio (1/4 life-size). Sigma and Tamron offer 70-300mm zooms that are an exception, focusing close enough to produce a 1:2 (1/2 life-size) reproduction ratio.

Even if you're not interested in true macro shooting, consider the minimum focusing distance when comparing potential lens purchases. If you like to photograph flowers and other small subjects, a 70-300mm zoom that focuses down to 3.1 feet at 300mm will make you a lot happier than one that focuses down to only 4.6 feet or down to 3.1 feet but only at the 70mm setting.

Lens Stabilization

Long lenses magnify the effect of camera shake, so it's best to mount them on tripods. If you prefer to work handheld, you might consider a lens that has a built-in image stabilizer. Stabilizers compensate for camera shake, allowing you to get sharp images handheld at two or three shutter speeds slower than is possible without a stabilizer. The Canon Image Stabilization lenses are identified by "IS" in the designation and currently include the EF 70-200mm ƒ/2.8L IS USM, EF 70-300mm ƒ/4-5.6 IS USM, EF 70- 300mm ƒ/4.5-5.6 DO IS USM and EF 100-400mm ƒ/4.5- 5.6L IS USM tele-zooms. Nikon offers the 70-200mm ƒ/2.8G ED-IF AF-S VR Zoom-Nikkor, "VR" standing for "Vibration Reduction." Sigma's entry is the APO 80-400mm ƒ/4.5- 5.6 EX OS (for Optical Stabilizer) zoom.


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