922. The size of the photographic image of an object depends first upon the size of the object; second, on its distance from the lens; and third, on the focal length of the lens employed.

923. The size of the object to be photographed is unalterable; if, however, it is wished to take different sized pictures of it with one and the same positive lens, then the object must be photographed at different distances. If the distance from an object is considerable, then pictures of different sizes of this object can only be taken by employing lenses of different focal length.

924. The size, weight and price of a complete photographic outfit grow with the increasing length of focus of the lens, so that the limit is soon reached beyond which the focal length cannot be considerably extended. This difficulty is readily overcome by substituting a telephoto lens, when it is no longer necessary to alter the position, nor to use lenses of different focal lengths, to obtain pictures of different sizes of one and the same object, from the same standpoint.

925. In the same manner as a telescope helps the human eye to see distant objects larger and clearer, so the telephoto lens, which is a photographic telescope, helps to increase the size of the image projected on the plate, which increase is effected by the addition of a negative lens to the positive elements of the lens in use.

926. Every possessor of a good photographic lens can convert it, by the addition of a telenegative with a teletube, into a telephoto lens, the positive lens being unaltered, and still available for ordinary use.

927. Telephoto lenses are lighter, more convenient, and less costly than positive lenses of correspondingly long focus. They give pictures of almost the same sharpness and clearness as ordinary lenses, but cover a smaller angle, and have less rapidity, the latter becoming less the greater the magnification obtained. On the other hand, by using the telephoto lens pictures which require a long focus lens can be taken with small cameras with short bellows, an advantage unobtainable by any other method.

928. "With a telephoto attachment, therefore, we obtain the advantages of a lens of long-equivalent focus without the need of a corresponding increase of bellows extension. As will be seen later, this equivalent focus, and therefore the magnification, can be varied. By the use of a telephoto lens objects situated at a remote distance or, from their position, inaccessible to the photographer with his ordinary outfit, can be faithfully recorded without difficulty. With it can be photographed the capitals of columns, carving or inscriptions, for which the camera without the attachment would be useless. The landscape worker, without moving his camera, can determine the amount of subject he will include upon his plate, can take mountain ranges, inaccessible peaks, and many a gem of scenery, which from the nature of its surroundings, can only be taken from a distance. The naturalist, perhaps, is even more indebted to

Telephotography 0600131

Illustration No. 92 the telephoto lens, for by its means large pictures of the most timid animals can be taken. Not only is the telephoto lens of value for distant objects, but for subjects near at hand it is equally useful, as it is only by use of the telephoto lens that objects can be reproduced in anything like natural sizes with due regard to their proportion, and without distortion or exaggerated perspective.

929. There is another important advantage of the telephoto lens. If we have to reproduce an object in exactly-full size, we, of course, measure our image on the screen, and compare this measure with the original. If we measure the image given by the positive lens on the screen, we find that only the parts lying in a certain plane are correct, those lying in other planes being either larger or smaller than the original. In the image taken with a telephoto lens we can compare the size of any part of the image with the original, which not only facilitates the working considerably, but insures a reproduction of exactly the same dimensions.

Telephotography 0600132

Illustration No. 95.

Illustration No. 96 Voigtländer Telephoto Lens

Illustration No. 96 Voigtländer Telephoto Lens.

930. The telephoto lens is essentially a negative lens adjusted in a long tube, which latter is attached to the front board of the camera, the opposite end being threaded to receive the photographic lens, which latter is termed a positive lens. The adjustable tube usually has a scale to indicate magnifications. In Illustration No. 92 is shown the optical construction of the Bausch & Lomb telephoto attachment, while Illustration No. 93 shows the exterior view of this instrument. Illustration No. 94 shows the Goerz telephoto lens and mount, in the front of which has been fastened the positive lens. The Dallmeyer telephoto lens is shown in Illustration No. 95, while Illustration No. 96 shows the Voigtländer telephoto instrument.

931. A telephoto optical demonstration is shown in Illustration No. 97. This picture was taken with a Bausch & Lomb-Zeiss Protar, Series VIIA, with a high power telephoto attachment magnifying 5 1/2 diameters. The small picture in the upper corner shows the same scene as it appears in a photograph made with the same lens without the telephoto attachment.

932. The following table will give you an idea of the distance required for certain size plates from the front board of the camera to the ground-glass, for various magnifications :

933.

DISTANCE FROM THE FRONT BOARD TO THE GROUND GLASS.

Size

Equivalent Focus of Photo

Lens

Magnification Resulting Focus when used with Tele-Photo Attachment

8

4

5

6

7

8

Inches

Inches

Inches

Inches

Inches

Inches

Inches

Inches

4x5

6 1/2

6 3/8

9 5/8

12 7/8

16 1/8

19 3/8

22 5/8

5x8

8 3/4

8 1/2

13

17 1/2

22

26 1/2

31

6 1/2 x 8 1/2

11 1/2

10 1/2

16 1/2

22 1/2

28 1/2

34 1/2

40 1/2

8 x 10

13 1/2

13 1/2

20 1/2

27 1/2

34 1/2

41 1/2

48 1/2