Monday, January 29, 2007
Lens Buying Guide
Everything you need to know about focal lengths, maximum apertures, new technologies and more!
INTERNAL FOCUSING. Internal focusing (IF) does exactly what its name implies, achieving focus by moving the rear or middle groups of lens elements contained within, rather than by moving the entire lens barrel and all the elements as a single unit. Compared to the latter system, IF saves weight, especially with big telephoto lenses. Internal-focusing lenses also can be made more compact, and because of the lighter weight of the few moving components, autofocus is faster. Unlike some simple zoom lenses, the fronts of IF lenses don't rotate, which is important for users of polarizers, graduated ND filters or any other accessory that depends on a constant position.
DIFFRACTIVE OPTICS (DO) AND PHASE-FRESNEL LENSES. These advanced designs would need a short course in physics to explain fully. The quick version is that lens designers have removed about a third of the size and weight of a typical lens by incorporating a high-tech optical element that uses concentric diffraction rings to focus light. The weight savings are due to the special element's distinct ability to combat spherical and chromatic aberrations, which eliminates the need for several conventional lens elements.
STABILIZATION SYSTEMS. A photographic rule of thumb says that the slowest shutter speed to use when you shoot handheld is the one nearest the reciprocal of the focal length you're using; with a 50mm lens, you'd shoot at 1⁄60 sec., with a 135mm lens, you'd shoot at 1⁄125 sec., and so on. Image-stabilizing systems give you sharp images at shutter speeds slower than you could otherwise use, typically two to three stops slower.
The stabilizers work by shifting some of the lens elements to compensate for camera movement, thereby keeping the image motionless on the image sensor. Canon's Image Stabilizer (IS), Nikon's Vibration Reduction (VR) and Sigma's Optical Stabilizer (OS) are all examples of this technology. Konica Minolta's Anti-Shake system provides similar results, but because the system shifts the image sensor rather than the lens elements, Konica Minolta lenses don't need stabilization technology.
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