Lenses are the eyes of a camera. They allow you to see close up, middle distance, or far away. How do you decide what lenses you need? That depends on what kind of photos you want to take. Do you mostly take people pictures? For portraits, you need a 90mm or 105mm lens with a wide aperture to eliminate backgrounds. Are you a bird photographer? You'll need a long (200mm+) telephoto (tele) lens. If you like landscapes, you'll probably want at least a medium (28-35mm) wide-angle lens. New SLR cameras often come standard with a 28-70mm or 28-80mm medium-range zoom. These focal lengths cover many picture taking situations, such as group and individual portraits, travel pictures, or party pictures. What they do not do is allow close-ups, wide-angle pictures, or photos of distant objects, such as a bird in a tree or a slide into second base.
For most situation, an ideal lens outfit consists of three lenses: a wide angle covering about 19mm to 35mm; a medium-range zoom in the 28-105mm range; and a long tele from 100-300mm, plus a 1.5 or 2X teleconverter, which lengthens any lens by 50 to 100 percent. Lens designers have developed very lightweight designs, so it is not hard to carry this ensemble in a camera bag.
There are lenses for virtually any specialized need, and in previous years, single focal length lenses were considered the only way to go because of superior optical design. But zooms are the lenses of today and few photographers, pros included, use single focal length lenses as often as they used to. For sports or nature photography there are 500mm and 1000mm lenses. But there are also zooms with a range of 400-800mm, and in at least one instance, 1200-1700mm (this lens is neither light nor cheap, and is obviously not an everyday item). For close-ups of insects and flowers, a 100mm macro allows you to shoot close at 1:1 image reproduction. There are wide angles down to 14mm, even fisheyes that give a distorted but interesting view in which the photographer needs to be careful to keep his or her feet out of the shot.
If you want to carry just one lens, you can get excellent pictures with the basic 28-70mm lens as long as you shoot mostly snapshots. However, a 28-200mm lens gives you the same view at the short end, especially useful for group photos indoors, as well as much more reach at the long end, plus the perfect portrait range of 90-105mm.