A digital telephoto lens is a type of photography lens used in conjunction with a digital camera. This lens is used for viewing objects far away from the person taking the photo, and is much shorter than the typical amount of space necessary to capture such images. The lens is shortened by using a series of refractory surfaces known as a telephoto group that are mounted within the lens body. A digital telephoto lens is specially designed for use with a digital camera, which uses a digital sensor rather than film to record a photo.
If a lens is designed to focus on objects far away, but it is not a digital telephoto lens, that lens is likely to be quite long to allow the light entering the unit to refract properly for making an imprint on the camera's sensor. A telephoto lens eliminates some of this length by using a series of specially designed lenses within the lens body. If, for example, a lens were to have a focal length of 300 mm, the lens itself would need to be 300 mm away from the digital camera's sensor. By using a telephoto group of lenses within the body, the image can be altered to essentially extend the light coming through the outermost lens.
The focal length of some digital telephoto lens models can also be changed by turning an adjustment wheel on the outside of the lens body. This is known as zooming; if the lens does not zoom, it is sometimes called a prime lens or a fixed focal length lens. Zoom lenses are very popular for their versatility in a variety of situations, but they may not always be the best choice for certain applications, as they may not be able to pick up on as much light as fixed focal length lenses.
Many features available on a digital telephoto lens were not available on older versions of the lens. Image stabilization features, for example, will prevent blurring of the final image due to slight shaking from the photographer's hands, the wind, or other pressures on the camera set-up. Auto-focus features allow the digital telephoto lens to hone in on an object clearly, or focus, without the photographer having to make manual adjustments to the lens. By simply depressing the shutter button halfway, the lens will be activated to focus on an object.