telephoto lens

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A camera lens used to produce an image of a distant object without the need for a lens of very long focal length (which would make the camera similar to a telescope). A telephoto lens consists of a combination of a converging lens and a weak diverging lens (nearest to the film). This combination of lenses has the same effect as a single lens with a focal length fc, a long way from the film. The ratio fc/fb, where fb is the distance between the film and the nearer lens, is called the telephoto magnification.

In shooting films and videos, the camera is often required to move from a distance shot to a close-up without the image losing focus. This can be achieved with a zoom lens, consisting of a combination of lenses that allow for a change of focal length, with the telephoto magnification varying in value from nearly one to a much higher value.

Subjects: Physics.

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