A telephoto lens is a camera lens designed to enable people to take long focal length pictures using a lens with an actual length which is shorter than the focal length. For example, a 400mm telephoto lens is not in fact 400mm long, unlike a conventional 400mm lens. There are a number of reasons why people might want to use this type of lens, ranging from the need to have a camera which is easier to handle but still capable of long focal length images to a desire to take advantage of specific traits of these lenses.
When people hear the phrase “telephoto lens,” they often think of an extremely long camera lens. Some telephoto lenses are quite long, but this is not necessarily so, and long lenses aren't always telephoto. This term refers to a lens of a very specific construction, not a particular length.
In a telephoto lens, there is a lens element at the front of the camera just as there is with other cameras, but there is a second element in the back of the lens in front of the film which acts to magnify the front lens. This second element effectively increases the focal length of the camera by making it seem as though the first lens is further away than it really is. Historically, a telephoto effect was sometimes accomplished with the use of lenses positioned in front of an optical instrument, but today the structure is built right into the lens.
The primary reason to use this type of lens is to take photographs of objects at a distance. A lens with a short focal length cannot take good images of things in the distance. The longer the focal length, the more detail can be seen in the distance. With a telephoto lens, a certain amount of compression also occurs, with more objects fitting into the visual field. As focal length decreases, depth of field widens, which changes the look and feel of the image.
People often want to take images with a long focal length, as for example when someone on the shore wants to take pictures of boats. However, the lenses needed for such photographs can become very long and cumbersome. They make the camera hard to handle and they also put strain on the lens mount and can damage it. A telephoto allows photographers to have the best of both worlds in the form of long focal length and a shorter lens.