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Thermodynamics

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Telephoto Lens

To obtain an image of the object situated at a very long distance, a convex lens of large focal length should be used. In this case, to obtain a magnified image the distance between the lens and the photographic plate should be large. Thus a magnified image requires a long camera which may be unmanageable and inconvenient. To avoid this difficulty a telephoto lens is used. It consists of two lenses mounted in a tube. The tube is fixed in the camera in place of a single lens. One of the lenses is convex and the other is concave. Each lens is made achromatic.

The concave lens L₂ is placed in the original position of the lens in the camera, whereas the convex lens L₁ is placed in the front of the lens L₂. The combination acts as a convex lens.

If ƒ₁ and ƒ₂ are the focal lengths of the lenses L₁ and L₂ and d is the distance between them, then

Consider that the rays from the distant object after passing through the lens L₁ meet at F₁ but the lens L₂ placed in the path diverges the rays and finally the rays meet at F₂.

The combination of lenses, L₁ and L₂ has formed the final image at F₂ which can be formed a single convex lens of focal length DF₂ placed at D. Thus we find that the telephoto lens acts as a covering lens of large focal length. It also avoids the use of very long camera box and can be used for taking highly magnified images even with a small camera.