Learn about telephoto lenses.
A telephoto lens has a much longer focal length than a standard or wide-angle lens. In other words, the distance between the lens and the negative is much greater. This longer focal length allows telephoto lenses to produce images in which subjects appear larger or closer than they actually are. The short focal length of the telephoto lens produces low depth of field and a narrow angle of view.
Focal Length and Image Size
Telephoto lenses often have focal lengths as great as 200 mm to 600 mm long. When the photographer depresses the shutter button, light passes through the lens and onto the negative at the back of the camera. Light passing through the top of the lens makes its way in a straight line to the bottom of the negative, and light passing through the bottom of the lens makes its way to the top of the negative. Thus, an upside-down image is produced on the negative inside the camera. Because the light has to travel farther through a telephoto lens than through a standard lens, the image projected onto the negative is enlarged, and the subject appears closer.
Angle of View
Standard camera lenses with focal lengths between 40 mm and 60 mm have about the same angle of view as the human eye. Telephoto lenses, however, with much longer focal lengths, have a much narrower angle of view than the human eye. So the telephoto lens cannot capture as much of a subject in a single image as a standard or wide-angle lens can capture. Because the light has to travel farther before hitting the negative at the back of the camera, only light reflecting off of the center of the subject matter that's aimed at will reach the negative. Thus, while photographers can get closer to their subjects by using a telephoto lens, they cannot include as much of the subject's surroundings or environment in the image.
Depth of Field
Telephoto lenses produce much shallower depth of field than standard or wide-angle lenses. This means that less of the foreground and background surrounding the subject will appear in focus. Telephoto lenses require a wider aperture to allow light to pass through the lens to the negative. The wider the aperture used to create an image, the shallower the image's depth of field. For example, an image of a pitcher on the pitcher's mound taken with a telephoto camera will compress the space of the foreground and background, making the distance between the pitcher and home plate look much shorter than the 60 feet, six inches we know it to be.
Fixed Focal Length vs. Zoom
Photographers can choose whether to work with a zoom telephoto lens or a telephoto lens with a fixed focal length. You can find telephoto lenses with fixed focal lengths ranging from 200 mm to 600 mm or greater, and zoom telephoto lenses with focal length ranges as wide as 55 mm to 400 mm. These zoom lenses allow you to switch quickly between focal lengths for larger or smaller image size, narrower or wider angle of view, or shallower or deeper depth of field.
Common Uses
Telephoto lenses work particularly well for sports and wildlife photography. The extreme close-up capabilities of the telephoto lens allows the photographer to position herself far from the subject. Furthermore, the shallower depth of field produced by the telephoto lens can isolate the subject, whether a pitcher or a distant zebra, by blurring the foreground and background. Thus, the viewer focuses only on the subject without being distracted by a foreground that appears sharply in focus. Telephoto lenses can also work well in landscape photography, making a sunset or the moon appear much larger and closer than it would to the human eye.
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