Stork's Nest—Telephoto Lens.
TELEPHOTOGRAPHY
By Dwight L. Elmendorf
Illustrated by the Author's Photographs
JUST when the telescope was invented is not known, but it is certain that Galileo was the first to direct his toward the heavens early in the seventeenth century. His instrument consisted of a long tube with a convex lens at one end and a concave ocular at the other. A modified form of this instrument still obtains in the ordinary opera and field glasses, which are binocular Galilean telescopes; and a single barrel of a field-glass is practically the telephoto lens of to-day.
Whenever anything is so far away that we cannot see it distinctly, we make use of a field-glass or telescope, which produces a magnified image of the object so that we are able to perceive what the unaided eye could not. In a similar manner the telephoto attachment enlarges the image formed by the ordinary lens in the camera. To produce on a photographic plate an image that fairly resembles what our eyes see, requires a lens of much longer focus than is generally used, and a camera that would permit the use of such a lens would be unwieldy and too cumbersome for a peripatetic photographer, and simply impossible for a mountain-climber. The telephoto lens overcomes this difficulty by producing the effect of a lens of long focus in a very compact camera.
It would be interesting to know who first applied this form of lens to a camera for the purpose of photographing distant objects. In 1890, while experimenting with the lenses from an old field-glass, I discovered that a dim yet distinct image of St. Patrick's Cathedral spires was formed in my camera, although the Cathedral was eighteen blocks away. After making several exposures with this combination of lenses I became convinced that with lenses of the best possible optical construction wonderful results might be attained. Having previously purchased a telescope with a three-and-a-half inch lens of sixty inches focus (with the idea of attaching it to a long box-camera as a photographic lens for the purpose of making photographs of distant terrestrial objects, as astronomers photograph heavenly bodies). I found that the field-glass combination of lenses yielded an image nearly as large as that produced by the telescope lens, and that too with a camera only one third the length of the other.
Milan Cathedral from Opposite Corner of Piazza.
Becoming deeply interested in this line of investigation I called upon a celebrated lens maker in London and learned that he had manufactured what he called a "Compound Telephoto Lens" consisting of a portrait lens with a small negative or concave lens adjusted at a suitable distance back of it. This instrument was too large and cumbersome for my small camera, and shortly afterward a negative lens, with a rack and pinion mounting, was manufactured of such a size that it could be attached to any fine rectilinear lens of suitable focus, although in some cases special corrections are necessary.
This is called the "Telephoto Attachment," and was employed in making the telephoto illustrations here shown. The tube is 3¼ inches long and 1½ inch in diameter. When this lens is attached to the ordinary lens the time of exposure is necessarily increased, because only a few of the rays of light which diverge from the positive or ordinary lens pass through the negative lens to the plate. This is a serious drawback, for it not only debars one from using it upon moving subjects, but also increases the liability of the image to be blurred by vibrations of the camera. In order to obtain the best results the camera must be very rigid. Most of the cameras and tripods of to-day are too light and unstable for telephotography.