Tuesday, June 8, 2010
One Lens To Shoot Anything
Big-range zooms (10x and more) aren’t just for travel anymore. With good sharpness and contrast across their focal lengths, today’s models are some of the most advanced optics on the market and they’re designed for digital.
Most superzooms are varifocal lenses, which don’t maintain focus as they’re zoomed. This usually isn’t a problem when autofocusing, as the AF system automatically handles it, but it’s something to keep in mind if you focus manually. With a true zoom, you can focus manually at the longest focal length for a magnified view of the subject, then zoom out to the desired wider view for the shot. With a varifocal lens, you can’t because focus will change as you zoom out; you’ll have to focus at the desired focal length for the shot.
Another superzoom variable is the maximum aperture. As mentioned, a typical 18-200mm lens has a maximum aperture of ƒ/3.5 at the widest focal length and a maximum aperture of ƒ/6.3 at the longest. (These carry through the aperture range; stopped all the way down, the 18-200mm ƒ/3.5-6.3 zoom has an aperture of ƒ/22 at 28mm and ƒ/40 at 200mm.) Through-the-lens metering automatically handles this, but you’ll have to keep it in mind if setting exposures manually via a handheld meter or the Sunny 16 rule.
Yet another variable is the fact that with most superzooms, the focal length decreases as you focus closer, such that at the minimum focusing distance, the 200mm setting may really be only 140mm or so. This means you don’t get the magnification you’d expect from a 200mm lens focused at 17.7 inches. Some people get upset when they learn this, but it’s not that big a deal. First, no 200mm prime lens (except a true macro one) can focus anywhere near 17.7 inches; most can’t focus closer than five feet (60 inches—three times farther away). Second, the superzooms still give you considerably bigger images of flowers and bugs in your close-ups than a 200mm prime lens can—140mm at 17.7 inches produces more magnification than 200mm at 60 inches. Bottom line: Most 18-200mm superzooms will get you close enough to produce an image one-quarter life-size at the film plane, while most 200mm prime lenses (true macro lenses excepted) will only take you down to around one-sixth life-size.
Internal Focusing. All the 10x-plus superzooms feature internal focusing. This means elements inside the lens are shifted to adjust focus, rather than physically extending the lens to adjust focus. Advantages of internal focusing are many. Since fewer and smaller elements have to be moved, autofocusing is quicker. The physical length of the lens doesn’t change during focusing, so balance is better. The front element doesn’t rotate during focusing, so orientation-sensitive lens attachments like polarizers and graduated filters retain their orientation during focusing. Internal focusing also allows for much closer minimum focusing distances (although the focal length often decreases somewhat at the closest focusing distances), generally providing reproduction ratios of around one-quarter life-size at the longest focal-length setting, great for flower close-ups. (Note that while the lens doesn’t change physical length during focusing, superzooms do increase in physical length when zoomed to their longest focal lengths.)
Stabilization. Many superzooms feature built-in image stabilization (Canon’s pioneering stabilization is called IS, for Image Stabilizer; Nikon’s is VR, for Vibration Reduction; Sigma’s is OS, for Optical Stabilizer; and Tamron’s is VC, for Vibration Compensation). With these lenses, a group of elements is shifted to counter handheld camera shake, and this is a terrific feature if you shoot handheld, especially at longer focal lengths or in dim light. Bear in mind that stabilization will compensate for camera movement, but not subject movement—moving subjects will come out blurred at slower shutter speeds unless you pan the camera to track their motion. Olympus, Pentax and Sony offer DSLRs with built-in sensor-shift stabilization, which moves the sensor itself instead of lens elements to counter camera shake. This has the advantage of working with any lens you attach to the camera and the drawback of stabilizing only the recorded image, not what you see in the viewfinder. If you use a stabilized superzoom with an Olympus, a Pentax or a Sony body that has sensor-shift stabilization, switch off either the lens or body stabilizer—don’t use both stabilizers simultaneously.
Special Elements. Wide-angle lenses tend to suffer from barrel distortion, spherical aberration and chromatic aberration. Long lenses tend to suffer from pincushion distortion and various aberrations. Correcting these with standard lens elements is difficult, if not impossible, even for a single focal length. So lens manufacturers use special elements in combinations to correct them amazingly well. ED, LD and SLD elements have unusually low dispersion characteristics, minimizing chromatic aberrations. Aspherical elements help focus light rays from the edges of the lens at the same plane as light rays coming in through the middle. These and other exotic elements allow designers to produce lenses that perform well through a wide range of focal lengths, are remarkably compact, focus amazingly close and cost surprisingly little. All superzooms employ a number of these special elements.
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