Knowing How Digital Camera Lenses Work

When shopping for a digital camera, people often overlook the camera lens. Serving as your camera's "eye," the lens determines what your camera can see — and how well that view is transmitted to the CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) chip for recording. The following sections explain the basics of how digital camera lenses work.

Focal length

Different lenses have different focal lengths. On a digital camera, focal length measures the distance between the lens and the image sensor, measured in millimeters.

Focus on these focal-length facts:

• Focal length determines the lens's angle of view and the size at which your subject appears in the frame:

• Wide-angle: Lenses with short focal lengths are known as wide-angle lenses. A short focal length has the visual effect of "pushing" the subject away from you and making it appear smaller. You can fit more of the scene into the frame without moving back.

• Some wide-angle lenses cause a problem known as convergence, a distortion that makes vertical structures appear to lean toward the center of the frame. If you plan to do a lot of wide-angle shooting, take test pictures to check for this issue before buying a camera.

• Telephoto: Lenses with long focal lengths are called telephoto lenses. A long focal length seems to bring the subject closer to you and increases the subject's size in the frame.

• "Normal": On most point-and-shoot cameras, a focal length in the neighborhood of 35mm is considered a "normal" lens — that is, somewhere between a wide-angle and a telephoto. This focal length is appropriate for the kinds of snapshots most people take.

• Cameras that offer a zoom lens enable you to vary focal length. As you zoom in, the focal length increases; as you zoom out, it decreases.

• A few cameras offer dual lenses, which usually provide a standard, snapshot-oriented focal length plus a telephoto focal length. In addition, some cameras have macro modes, which permit close-up photography.

To get a visual perspective on focal length, take a look at Figure 1. Here, you see the same scene captured at four different focal lengths. As the focal length increases, the lens can capture less and less of the landscape.

Optical versus digital zoom

Zoom lenses give you a closer view of far-away subjects. A zoom lens is especially useful for travel photography and is also good for portraits or still-life shots in which you want to shoot a subject without including a large amount of background.

If a zoom lens is important to you, be sure that the camera you buy has an optical zoom. An optical zoom is a true zoom lens. Some cameras instead offer a digital zoom. When you use a digital zoom, the camera enlarges the image area at the center of the frame and trims away the outside edges of the picture. The result is the same as when you open an image in your photo-editing program, crop away the edges of the picture, and then enlarge the remaining portion of the photo. Enlarging the "zoomed" area reduces the image resolution and the image quality.

Focusing aids

Some cameras have fixed-focus lenses, which means that the point of focus is unchangeable. Usually, this type of lens is engineered so that images appear in sharp focus from a few feet in front of the camera to infinity.

Many cameras enable you to adjust the focus point for three distances: macro mode for extreme close-ups, portrait mode for subjects a dozen feet from the camera, and landscape mode for distant subjects.

Cameras with autofocus automatically adjust the focus depending on the distance of the subject from the lens. Most cameras with autofocus abilities offer a useful feature called focus lock. You can use this feature to specify the object you want the camera to focus on, regardless of the object's position in the frame. Usually, you center the subject in the viewfinder, press the shutter button halfway down to lock the focus, reframe, and then snap the picture.

Digital SLR (single lens reflex) cameras and a few high-end point-and-shoot models offer the option to switch from automatic to manual focusing, giving you complete control over the focus range. On SLRs, you adjust focus by twisting a manual focus ring on the lens barrel, just as with a film SLR camera. On the point-and-shoot models, you usually set the focus point a specific distance from the camera — 12 inches, 3 feet, and so on — via a menu on the LCD monitor.

Lens adaptability

Digital SLR cameras allow you the same lens flexibility you get with film SLR cameras. You also can add color and effects filters, just as you can with film models.

If you're a serious photography buff but not ready to invest in a digital SLR, you may want to buy a point-and-shoot model that accepts lens adapters and filters. Many cameras now sport lens designs that enable you to screw on wide-angle, fish-eye, or close-up lens adapters. Several third-party companies also offer adapters and filters for popular cameras.