Zoom needs could guide your choice of camera



An old friend who's buying a digital camera sent an e-mail with this question: "How much zoom do you really need in your life?"

My immediate thought was, "At our age, a lot more than we used to." But what my friend really wanted to know was whether to buy one of the new "super-zoom" digital cameras or stick with a less-expensive, traditional model with more modest zoom capability.

Questions like these are increasingly common as digital photography loses its mystery and buyers ask what a digital camera can do instead of how it works. Many customers are asking for cameras that can nail an ivory-billed woodpecker in a tree or reach across a soccer field for a sharp close-up of game action.

To deal with this issue, it helps to know a little bit about lenses - and about focal length in particular.

Technically speaking, the focal length of a lens is the distance from the center of the lens to the film plane (on traditional cameras) or to the charge-coupled device (CCD) that records the image on a digital camera. In practical terms, the focal length determines the angle of view and magnification of the image.

A lens that roughly matches the perspective of your eye is referred to as a "normal" lens. A lens with a shorter focal length, known as a wide-angle lens, crams more into the picture. But most objects will appear smaller than they do in real life, and objects in the foreground will be somewhat exaggerated - particularly people's noses when you get close.

A lens that narrows the field of view and magnifies a smaller part of the normal image is known as a telephoto lens. So-called "short" telephotos are good for flattering portraits, while a "longer" telephoto lens will bring faraway action much closer, like a pair of binoculars.

For the sharpest results, serious photographers use single-lens reflex cameras that accommodate separate, interchangeable lenses with different focal lengths. But they're expensive and a pain in the neck, even for pros.

Most consumer-oriented cameras, film and digital, rely on a built-in "zoom" lens that provides multiple focal lengths. These lenses are measured by the ratio of the "zoomiest" setting (with the greatest magnification) to the widest-angle setting, which takes in the largest area.

Typical consumer cameras have 3-to-1 optical zoom lenses, a measurement often abbreviated as 3X. If you're experienced with traditional 35-millimeter film cameras, a 3X zoom provides the equivalent of a traditional 35-mm to 105-mm zoom lens.

In human photographic terms, a 3X lens can zoom from a slightly wide-angle setting that's fine for groups of people indoors but not quite wide enough for good landscapes, to a short telephoto setting that provides flattering portraits of people from a distance of 6 to 10 feet, but isn't powerful enough to isolate sports action far from the sidelines.

For years, the main alternative to a 3X consumer zoom was the single-lens reflex camera with interchangeable lenses.

But manufacturers are turning out a new generation of consumer-oriented, super-zoom cameras with optical zoom ratios as high as 10- or 12-to-1. At the farthest extension, they can provide the pulling power of a traditional 400- to 500-millimeter lens - the kind sports photographers use.

Because they don't take interchangeable lenses, super-zooms are less expensive to manufacture than single-lens reflex models. But they cost more than standard 3X cameras. Besides the expense of the lens itself, most super-zooms use a mirror to provide a through-the-lens view of your subject, which increases the cost.

Bottom line: figure on $400 to start to for a 5-megapixel model and $800 to $1,000 for higher-resolution super-zoom cameras.

Most super-zooms are also considerably larger than consumer point-and-shoot models. They won't fit in a pocket - a serious consideration if size and weight are important when you're on vacation.

But the real problem with any zoom lens is that it's hard to take a sharp photo when the lens is fully extended - even on a bright day. Part of this is optics - the further out you zoom the lens, the less light it captures. So the camera's light meter compensates by decreasing the shutter speed.

This brings us to the second issue. When they're extended, zoom lenses not only magnify the image, but also exaggerate camera shake. The ideal shutter speed for a long zoom is 1/500th of a second or less. But the camera's light meter will often try to capture extended zoom shots at 1/60th of a second or more. The result: less-than-ideal photos.

To compensate for slow shutter speeds and shaky hands, many manufacturers offer image stabilzation - technology that almost imperceptibly moves the lens or the CCD to counter the camera's movement. This is very cool stuff - in fact, I believe it's accomplished by voodoo, but whatever the source, it can work amazingly well. As long as you can afford it, buy a camera with image stabilization - even on a 3X zoom model if you can find it.

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