Expand your creative options with the right lens--or lenses
Learn your lens lines
Knowing what type of lens works best for a given situation requires a bit of background information. Fortunately, there are only two specifications you really need to understand.
Focal length (expressed in millimeters): This indicates the angle of coverage, or how much of a scene a lens will take in. The longer the focal length, the closer the subject will appear. The shorter the focal length, the more of a scene is included. If you want to make distant objects appear closer, go with a longer lens, called a telephoto. If your desire is to get more into the picture, choose a wide-angle lens. If the lens is a zoom lens, the focal length is indicated as a range from the shortest to the longest, for example, 28-200mm. A wide-angle-to-telephoto zoom offers the best of both worlds, but later you'll see why a fixed focal length lens sometimes has advantages over zoom lenses.
Using a long telephoto or long zoom requires a high shutter speed and a steady hand--or better yet, a sturdy tripod. If you try to hand-hold a 300mm lens and shoot at 1/30th of a second shutter speed you are going to get a blurry image--guaranteed. The old (but accurate) rule of thumb is to use a shutter speed that is "1 over the effective focal length," or in other words, shoot a 300mm at 1/300th second or faster. Remember, we're talking "effective focal length" here--see sidebar. The other way to combat camera shake is to use lenses (or cameras) that have Image Stabilization or Anti Shake technology. It really works and will generally remedy even a bad case of the shakes.
Lenses made to fit film SLR cameras generally will work on digital SLR cameras. However, there is an entirely new crop of lenses built especially for DSLRs and they will not work with film cameras. This is true because the difference between the size of the imaging sensor and the size of one frame of 35mm film. For the same reason, a 200mm lens has an effective focal length of 300mm (or 320mm) when used on a DSLR. Some digital-specific lenses have an improved antireflection coating on their rear elements because digital imaging sensors reflect more light than film does. The coating helps eliminate flair. Other digital lenses are optimized in other ways, depending on brand.
Aperture (expressed as the "f/stop"): The f/stop number indicates how much light can pass through the lens. Small numbers, like f/2 or f/2.8 pass more light than large numbers like f8 or f22. If you keep in mind that these are ratios (like fractions), it will be easier to understand. Obviously, if a lens is capable of transmitting a large quantity of light it can be used in situations where the light level is lower. But equally important is the fact that the camera's viewfinder will be brighter, too.
Lenses that have larger apertures are said to be "fast." That's because, when set to their largest apertures, you can choose a faster shutter speed. The lenses are also better suited for use in low light without flash. Buying tip: Fast lenses tend to be more expensive, especially fast zoom lenses. A 28-70mm f/2.8 lens can easily cost five to ten times what you'd pay for a 28-70mm f/5.6 lens, and is bigger and heavier. Is it worth the extra cost and weight? That's up to you!
Lenses made by camera manufacturers will operate only on their camera brand. You cannot use a Nikon lens on a Pentax, for instance. There are, however, many independent companies that manufacturer lenses in different "mounts." The big three are Sigma, Tokina, and Tamron. If you own a Canon, for example, you must use a lens that has a Canon mount but it does not necessarily have to be a Canon-brand lens.
Why go with an independent manufacturer's lens? They generally cost less. The trade-off? Some lenses may not produce the same quality as the camera maker's lens, which is precisely matched to the camera. This is more important on pro-oriented lenses.
DEPTH OF FIELD
There is one more thing you should know about lenses. Depth-of-field describes the distance behind your subject and in front of your subject that will be in focus at the same time. You've seen those dramatic portraits where the background looks all melted away? The photographer carefully controlled the depth-of-field.
Wide-angle lenses deliver more depth-of-field than telephoto lenses. Small f/stops, like f16 or f22, deliver more depth-of-field than large f/stops like f2.8. If you shoot a wideangle lens at f16, darn near everything is going to be in focus, regardless how you have the camera set, and whether you like it or not. (This is how inexpensive, non-focusing cameras work.) On the other hand, to get that dramatic portrait described above--the one that has very limited depth-of-field--use a long focal length lens and shoot at the largest opening.
FIXED FOCAL LENGTHS
Not all lenses zoom. Very long (400mm or longer) telephoto lenses, macro lenses and several other types are often fixed focal length lenses. If you need a lens for a more specific kind of photo, such as extreme close-ups of small subjects (macro), up-close views of sports or wildlife (long telephoto) or portraits (short telephoto), a fixed focal length lens may be best because it offers the largest aperture at the desired focal length for less money. Also, if size and weight are a concern, be sure to check out the non-zoom lenses--they are generally more compact.