Wednesday, November 1, 2006
Going Tele: A Complete Guide
What you need to know about buying and shooting with long lenses
Focal Length Vs. Format
Whether a given focal length is telephoto, normal or wide-angle depends on the camera format with which it's used. On a 35mm camera, a 200mm lens is quite telephoto—it provides a big image of the subject in the image frame (four times the size produced by the format's normal 50mm lens) and has a fairly narrow angle of view. The smaller image sensor of an APS-C sensor digital SLR crops in more on the image formed by any lens; a 200mm lens on an APS-C D-SLR will produce about the same image size within the frame and the same cropping as a 300mm lens on a 35mm camera.
Conversely, the larger image size of a 6x7cm. medium-format camera "sees" more of the image produced by any lens, so the 200mm lens has a wider angle of view (about equivalent to that of a 115mm lens on a 35mm camera), and the subject's image is smaller within the frame. A 4x5 view camera's big 4x5-inch image area sees even more of the image produced by the lens, so the 200mm lens is barely telephoto, equivalent to about a 58mm lens on a 35mm camera. The diagram above makes this easier to comprehend.
Prime Or Zoom?
All other things being equal, a fixed-focal-length lens probably is a little sharper than a zoom that goes out to the same focal length, simply because the zoom has to be optimized to perform at a whole range of focal lengths, not just one. The fixed-focal-length lens is also lighter because fewer elements are needed to produce one focal length efficiently. But today's major-brand tele-zoom lenses are quite sharp (some sharper than older prime lenses, due to use of modern design, manufacturing and materials) and not all that bulky; in fact, we now have zooms that go from wide-angle to supertelephoto (28- 300mm), yet are less than 3.5 inches long (when not zoomed all the way out).
The tele-zoom offers the versatility of a range of focal lengths in a single unit. This facilitates changing framing when you can't easily move toward or away from your subject, and it means fewer lens changes in the field—which in turn means less chance for dust to settle on your D-SLR's image sensor. Another advantage is that for those on tight budgets, you can get a longer focal length for less money in a zoom. There are a number of major-brand 70-300mm and 75-300mm zooms on the market for less than $300, while no major-brand fixed-focal-length 300mm lenses are available near that price. Sigma and Tamron offer tele-zooms that go out to 500mm, yet sell for less than $1,000,far less than the cost of a good fixed-focal-length 500mm lens.
Most prime telephoto focal lengths are available in superfast and slower versions. For example, I do most of my shooting with a 300mm ƒ/4 lens, while the same manufacturer also offers a 300mm ƒ/2.8. The faster lens permits shooting at a faster shutter speed in any given light level, provides quicker auto-focusing performance and offers a brighter viewfinder image for easier manual focusing. But the ƒ/2.8 lens also costs more than three times as much and weighs more than twice as much as my ƒ/4 lens, which is why I own and use the ƒ/4. Most pro wildlife and sports photographers use the superfast supertelephotos—300mm ƒ/2.8, 400mm ƒ/2.8 and 600mm ƒ/4—because they need the advantages cited above. These pros deal with the bulk by mounting the lens on a sturdy tripod and with the price by making money from the resulting photos. Most tele-zooms come in only one speed version, but when lens-speed options are available, consider the benefits versus the cost in weight and dollars of the faster versions. Again, faster lenses generally provide quicker autofocusing, important if you shoot action subjects.
Some lenses extend in physical length and rotate the front element as they're focused. This means the balance of the lens changes during focusing, and orientation-sensitive lens attachments such as polarizers and graduated filters will change their orientation as the front element rotates.
Internal focusing solves these problems and more. Only internal elements shift during focusing, so the physical length of the lens doesn't change, nor does balance. And the front element doesn't rotate, so polarizers and graduated filters retain their orientation. Because only smaller internal elements move, autofocusing is quicker with internal focusing lenses. Rear focusing provides all the advantages of internal focusing except the lens' physical length changes during focusing (albeit not as much as with front focusing).
With most AF lenses, trying to focus manually while in AF mode can damage the AF motor in the lens or camera body. There are exceptions, however, including most of the Canon USM (Ultrasonic Motor), Nikon AF-S (Silent Wave Motor) and Sigma HSM (Hypersonic Motor) lenses; check the instruction manual for your particular lens. In single-shot AF mode, this allows you to focus manually if the AF system has trouble with a specific subject or to fine tune focusing manually after autofocusing without fumbling for the AF/MF switch. In both single-shot and continuous AF modes, it allows you to prefocus manually at a ballpark setting, which speeds up autofocusing when you then press the shutter button halfway to activate the AF system because the lens won't have to search so far to find the focus point.
A lens' minimum focusing distance can be important. For instance, my 300mm ƒ/4 will focus down to 4.9 feet, while the same manufacturer's 300mm ƒ/2.8 lens won't focus closer than 8.2 feet. This allows me to get close-ups of subjects that would be too close to focus on with the faster lens.
When you can approach a subject closely, sometimes you can get a bigger image of the subject from a shorter lens than from a longer one. My 300mm lens at its 4.9-foot minimum focusing distance produces the same subject size in the image frame as a 600mm lens focused at 9.8 feet—but the 600mm lens from my camera's manufacturer won't focus closer than 18 feet. This means I can move in on, say, a spider, and get an image nearly twice as big in the frame with my 300mm lens as I could using that 600mm lens. Of course, when both lenses are used at the same distance (i.e., at any distance the 600mm lens can focus), the 600mm will produce twice the magnification of the 300mm lens.
There's a lot of focusing travel from inside five feet out to infinity with a long lens. To expedite autofocusing, many such lenses have a focus-range limiter switch. For example, my 300mm lens has a switch that allows me to limit focusing travel from infinity to 10 feet instead of the full infinity to five feet, so the lens doesn't have to hunt all the way down to five feet before racking back out toward a more-distant subject. If your lens has a focus-limiter, learn to use it; doing so will increase your percentage of sharp shots and decisive moments immensely.
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