One of the main attractions of using an SLR camera is the ability to change lenses. Not only will a different lens change how the image is cropped or how much of a scene is included, the perspective and optical characteristics change dramatically with a stronger wide-angle or telephoto lens. For example, not only will a wide-angle include more of the scene in a shot, it will change the perspective so something close to the lens appears large, compared to a more distant object. A telephoto has shallow depth of field, which you can use to your advantage by making the main subject dramatically stand out from the blurred, uncluttered background.

But with so many lenses available and with prices ranging from $100 to thousands of dollars, finding the best lens at first can seem to be a daunting task Here are some basic tips on lenses and the main points to keep in mind when selecting a new one.
MOUNTING STYLE
Each camera brand uses an exclusive mounting style, so a lens made for a Nikon will not fit a Canon or any other camera brand. The maker of your SLR camera produces many different lenses to cover all sorts of needs and situations, from ultra-wide angle to extreme telephoto. But independent makers such as Tamron, Sigma, and Tokina make excellent lenses as well. In many cases, lenses from independent manufacturers cost less than camera brand lenses. Also independent lenses often fill niches in zoom ranges, lens speed or other features not covered by the camera brand.
FIXED FOCAL LENGTH VERSUS ZOOM
The majority of the lenses on the market are zooms, due to their popularity and versatility. The millimeter numbers in a lens description indicate whether it is a zoom (2 number sets separated by a hyphen) or a fixed focal length lens. For example, an 11-18mm is a zoom, while a 24mm lens is a fixed focal length lens. What fixed focal length lenses give up in versatility, they usually gain in price, compact size, or fast optical speed.
FOCAL LENGTH RANGE
Quite likely you purchased an 18-55mm zoom lens with your digital SLR camera. This lens range covers a useful wide to short telephoto zoom range, is relatively inexpensive and produces good quality images. With most digital SLRs, 32mm is the "normal" focal length. At this setting, the view is similar to the human eye. The farther below 32mm you go (24mm, 18mm, 11mm, etc.), the wider the coverage and the smaller a main subject will appear. As you zoom farther above 32mm, the stronger the magnification effect will become and distant objects will be drawn in closer. If you purchased an 18-55mm or similar zoom with your camera, most often the second lens you may wish to consider is one to strengthen the telephoto power. A 55-200mm or 70-300mm will serve this purpose well. If you prefer to have just one versatile lens and not have to change lenses, you should consider an 18-200mm or 18-250mm. Either lens will be ideal for travelers and others who want to carry the least amount of camera gear possible and not be slowed down by switching lenses.
LENS SPEED
The "speed" of a lens has a large bearing on its physical size and how expensive it will be. A fast lens will have a small f-number and the glass elements will be wider in diameter to let in more light. This in turn allows a faster shutter speed to be used than with a slower lens. As an example, the Canon EF 70-200mm f2.8 IS USM Zoom is a full f-stop faster than the Canon EF 70-200mm f4 IS USM Zoom. But the faster lens weighs about twice as much as the f4 version and costs 40% more. The faster lens lets in more light for easier focusing and shooting in dim conditions, so the speed of the lens and your intended uses need to be considered.
IMAGE STABILIZATION
There are two types of image stabilization used in D-SLRs--optical, where lens elements shift to counteract camera movement, and sensor shift, which moves the imaging sensor in the camera body. Canon and Nikon use optical shift stabilization so some lenses have this feature, while others do not. At one time the image stabilization feature added quite a bit to the price of the lens, but this premium has come down considerably. Tamron and Sigma have introduced image stabilization to a few of their Canon and Nikon mount lenses, also. Current Pentax, Olympus and Sony SLRs have sensor shift image stabilization built into the body and any lens will couple to the stabilization feature.