Wednesday, November 1, 2006
Going Tele: A Complete Guide
What you need to know about buying and shooting with long lenses
Long lenses are particularly susceptible to chromatic aberrations, so manufacturers use special elements to compensate. Extra-low-dispersion elements correct these aberrations, which appear as color fringing in images made with uncorrected lenses. These elements are designated differently by different lensmakers; ED, LD, UD, SLD, ELD, SUD and HLD are some of the identifiers used. Fluorite elements also correct chromatic aberrations and are often used in conjunction with ED elements.
Aspherical elements correct spherical aberration, which appears as image softness and is especially evident in fast wideangle lenses and wide-range zooms. It's not a huge factor with tele focal lengths, and you won't find aspherical elements in many long lenses, just a few zooms.
Diffractive Optics (introduced by Canon in 2000 and indicated by DO in the lens name) correct chromatic and spherical aberrations with fewer, smaller elements, resulting in much more compact lenses.
Canon, Nikon and Sigma offer a number of telephoto and tele-zoom lenses with built-in image stabilizers that counteract camera shake. Canon lenses are designated "IS" (Image Stabilizer), Nikon lenses are "VR" (Vibration Reduction) and Sigma lenses are "OS" (Optical Stabilizer). I love stabilization because it gives me sharp shots at shutter speeds I couldn't handhold otherwise. For maximum sharpness, long lenses should be used on tripods, but if you have to (or prefer to) work handheld, a stabilized lens is worth its weight in gold (yes, even at today's gold prices!).
The new Pentax K100D and Sony DSLR-A100 (and the pioneering but out of-production Konica Minolta Maxxum 7D and 5D) provide in-camera stabilization: instead of shifting a group of lens elements to compensate for camera shake, they shift the image sensor itself. This has the advantage of providing stabilized shooting with all glass, not just special stabilizer lenses. On the downside, sensor-shift stabilization steadies only the recorded image, not what you see in the finder, so you can't see how well it's working as you shoot.
Tele-extenders (also known as teleconverters) are a cost-effective way to get long focal lengths. These little tubes fit between the lens and the camera body and increase a lens' focal length by the indicated amount—1.4x or 2x for the most popular extenders. As a bonus, they don't change the lens' minimum focusing distance in the process: if you attach a 2x extender to a 300mm lens that focuses down to 4.9 feet, you have a 600mm lens that focuses down to 4.9 feet.
The major drawback to extenders is that they reduce the amount of light transmitted to the film or image sensor—by one stop for a 1.4x extender and by two stops for a 2x extender. That means a 300mm ƒ/4 lens becomes a 420mm ƒ/5.6 lens with a 1.4x extender attached and a 600mm ƒ/8 with a 2x extender.
With many AF cameras, the AF system won't function with lenses slower than ƒ/5.6, meaning that with a 2x extender attached, that autofocus 300mm ƒ/4 lens becomes a manual-focus 600mm ƒ/8 lens—and the viewfinder image is pretty dark, making manual focusing in dim light difficult. But with practice, you can get the knack, and a 300mm ƒ/4 lens plus 2x extender costs less than half the price of the 600mm ƒ/4 lens. By the way, AF performance will slow noticeably even when a 1.4x extender is used because of the decrease in effective lens speed.
Tele-extenders do reduce image sharpness—minimally when a quality converter matched to the lens (or focal length) in use is employed, somewhat more when cheap or mismatched converters are used. I've seen some excellent results with my 300mm/2x converter combo; when I do get soft shots, it's because I misfocused manually, not because the lens/extender combo lacks sharpness.
Besides a relatively inexpensive way to acquire very long focal lengths, extenders provide versatility options. For example, you can buy one camera manufacturer's 400mm ƒ/5.6 supertelephoto lens for around $1,100. Or you could buy its 70-200mm ƒ/2.8 zoom lens for around the same price, add a $300 2x teleconverter, and turn the 70-200mm into a 140-400mm ƒ/5.6 lens. The 400mm ƒ/5.6 prime lens will probably be a tad sharper, but with the 70-200mm and 2x combo, you also get focal lengths from 400mm down to 70mm and a fast ƒ/2.8 maximum aperture from 70-200mm—very handy if you photograph a lot in dim light.
Perspective is really a function of camera position, not focal length. If you put the camera on a tripod and take a shot with a long lens, then remove the long lens and attach a short lens and take another shot of the scene, then blow up the shorter-lens shot so that it shows the same area of the scene as the long lens shot, the perspective will be identical. Why do some think increasing the focal length flattens perspective? Because we generally shoot from farther away when using a long lens, hence the "telephoto flattening" effect. TV news photographers covering stories on rush-hour traffic often use this technique to stack the cars up dramatically. You can also use the effect to enhance "heat mirage" distortion on hot days.
Depth Of Field
All other things being equal, the longer the focal length, the less the depth of field, and the wider the aperture, the less the depth of field. So depth of field is extremely limited with those superfast, very long focal-length lenses the pros use. When using very long focal lengths, it's best to focus carefully manually and close the aperture down a stop or two.You can use a long lens wide open to really isolate a subject from the background— the narrow angle of view crops out peripheral distractions, while the minimal depth of field throws background elements completely out of focus. It's best to focus manually when doing this; an AF system might not focus precisely where you want in selective-focus situations.
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