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A single-lens reflex (SLR) lens is a photographic lens designed to be mounted exclusively on an SLR camera. Featuring this type of camera mount, an SLR telephoto lens is one that makes use of the optical properties of a lens arrangement known as a telephoto group. This optical design allows a telephoto lens to achieve a greater degree of magnification within a more compact form.
The focal length of a camera lens is usually expressed in millimeters (mm). The greater the focal length, the greater the magnification of the subject. Long-focus lenses have large focal lengths, generally 135mm-500mm, and are used primarily to photograph distant objects. The physical length and focal length of these lenses are nearly identical. Such a lens can become cumbersome to manage as the focal length increases.
A telephoto lens is a long-focus lens with a unique optical design that offers increased focal length in a smaller, easier to use lens. This design consists of an initial group of lens elements that provides a positive focus to the incoming image, backed by a secondary group with a negative focus. The secondary set of elements is called the telephoto group. The effects of the two groups of optical elements combine to increase the effective focal length of the lens.
A principal feature of an SLR camera is the ease with which a photographer can change lenses. Manufacturers typically offer a wide range of lenses compatible with a particular SLR camera mount. The optical properties of each offer a specific benefit in composing and capturing an image under a given set of circumstances. When a photographer is unable to approach a subject closely, he or she is likely to employ an SLR telephoto lens. Another property of telephoto lenses, sometimes used in landscape photography, is the compression of depth.
There are drawbacks to the use of an SLR telephoto lens. Typically, lenses with longer focal length are prone to camera shake, especially when hand held. Camera shake can result in disappointingly blurry pictures, a problem photographers may often eliminate with the use of tripods. This can particularly be a problem with SLR cameras, as the viewing mirror is raised just before the shutter is engaged. There is also an increased chance of lens flare and certain kinds of distortion with their use.