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|THE STORY OF THE LENS||Photographing at a Distance|
|HE photographing of objects at great distances—the lonely mountain peak, architectural details, inaccessible bits of scenery, birds and animals in their native haunts—was all impossible until the introduction of the telephoto lens about thirty years ago. With the ordinary short-focus lens used at a distance, great mountain|
peaks seem only little hummocks of rock and snow. The richly sculptured
front of some Old-World cathedral is wholly lacking in the detail and
artistry for which it is chiefly interesting. The photographs of vivid
and realistic scenes of war which grip the observer with feelings almost
akin to actual participation are products of the telephoto lens or the
long focus cameras so much used in the Great War.
The telephoto lens is really a photographic telescope. It applies to the camera the same principle that is utilized in an ordinary opera glass. In the opera glass a concave or diverging lens is placed behind a converging objective and the result is a magnified erect image. Likewise the telephoto attachment consists of an adjustable tube having in the front the regular photographic lens and behind it at the other end a concave lens to diverge the rays of light and give a magnified image on the ground glass plate. Through a rack and pinion the separation between the two lenses may be varied and an optical system of variable focal length secured. This variation is from three to eight times that of the regular lens, and, therefore magnification of the image in the same ratio are obtained. In this wide range of focal lengths lies one of the chief advantages of the telephoto lens. In addition, the required bellows extensions are, in all cases, considerably shorter than the resulting focal lengths of the optical system would indicate, and, therefore, more compact camera outfits are possible. An important disadvantage of this lens is that a longer exposure is required. The necessary exposure varies as the square of the magnification. If the telephoto system is adjusted for an increase in the focal length of three times, an increase of exposure of nine times over that for the regular lens alone would be required.
As may be inferred from what has
already been said, the size of the image produced by a lens depends
upon its focal length. For distant objects the size of the image
increases directly with the increase in focal length. With the focal
length three times as great, the image will be three times as large.
But the intensity of light on the plate or film will be less; hence
the longer exposure required. Because of this impossibility of making
instantaneous exposures with the telephoto lens it was found necessary
for war work to develop high speed, long-focus lenses with cameras
having long bellows extensions. Taking photographs of troops in action,
or of belching cannon, or making exposures from an aeroplane were out of
the question with the telephoto lens. Great progress has been made in
the development of long-range cameras during the war. The French used
one type having a bellows extension of four feet. Such cameras would
formerly have been thought too cumbersome but the exigencies of war have
overcome this difficulty.
One of the chief advantages of long-range lenses is the freedom from distortion so prominent in views of large objects taken at close hand. With the telephoto or camera of long focal length, true perspective is obtained and that richness and vividness of detail so characteristic of modern panoramic photography. Much of the delight which we experience in viewing the splendid travelogue pictures of such noted lecturers as Burton Holmes and Dwight L. Elmendorf must be credited to these types of lenses. So, too, the charm attached to much of modern magazine illustrating and the realistic photography in the picture supplements of the Sunday newspaper. Surely the world owes much to the genius of those masters of optics who have brought to so high a degree of excellence the art of long distance photography.