This section is from the book "Telephotography: An Elementary Treatise On The Construction And Application Of The Telephotographic Lens", by Thomas Rudolphus Dallmeyer. Also available from Amazon: Telephotography and Telephotographic Lens.
It will have been observed from the premisses that if we include a foreground subject, such as a figure, under a large angle, we must, in order to obtain a sufficiently large image from a given standpoint, necessarily include a large angle of subject in the receding planes, which become dwarfed and insignificant in value. The further we remove the lens from the foreground of our subject, the smaller is the angle under which it is seen or photographed, and the receding planes have now greater importance than in the former case, although the whole is in a diminished scale. If by the Telephotographic lens we make the subject in the foreground of the same size as before at this greater distance, it is obvious that very much less subject will be included in the receding planes, and that they will be rendered in a larger scale than before, possessing due importance.
One frequently notices in a group of figures produced by ordinary photographic means, the rapid diminution in the scale of the images of persons in the receding planes of the group. This is easily obviated with the Telephotographic combination, by taking up a position in which the individuals forming the group are represented more nearly equal in size, and producing the required scale in the final image by the magnification obtained by the negative element of the system in conjunction with the requisite camera extension.
Ordinary common-sense considerations will enable the photographer readily to grasp the bearing of this subject. If we observe from a distance a moving object, such as a railway train, travelling directly in a line towards us, an advance in our direction of a hundred yards or so seems to make but very little difference in the size of the object itself; but as it continues to approach, equal increments of distance have a greater and greater effect upon its appearance in perspective drawing until, when it is so near that its distance from us is some small multiple only of its entire length, the perspective in which it is seen alters rapidly. The problem which the photographer has to solve in every case where there is considerable depth in his subject is: how far from the foreground his standpoint should be in order to maintain the perspective rendering in a degree which is not overdone in either of the two directions possible. On the one hand, by loss of perspective in taking the subject from too distant a standpoint, and thus diminishing the due effect of distance; or, on the other, by choosing too near a standpoint and so causing the receding planes of his picture to diminish too rapidly in scale, and thereby unduly extending the effect of distance. For the purpose of picture making, irrespective of truthful representation of the subject, the former error is the less objectionable. It must be noted that the perspective effect can be readily observed in the image formed by the positive lens alone, a small portion of the image near the centre of the plate only being considered. When the perspective effect of this small central portion is found satisfactory, the photographer will then increase its scale by means of the negative element of his Telephotographic system, and thus cause that small portion to fill the entire plate on the enlarged scale.
The Telephotographic lens applied to the hand-camera enables the photographer not only to obtain a larger scale in the image than is possible with a lens of ordinary construction, using the same camera extension, but also to produce the same scale given by the latter at a greater distance, with the accompanying improved perspective rendering. To give an example : suppose we take as our positive element a small portrait combination working at//3 of 5 inches focal length, and combine it with a negative element of 2 1/2 inches focal length, and set the screen at a distance of 5 inches from the negative element, we shall produce an image three times as large as that given by the 5-inch lens alone. In the case of ordinary studies of figure subjects we shall now be able to take up a standpoint three times as far as would be necessary to produce the same size of image with the 5-inch lens. The drawing will be vastly better, even in the foreground of the subject, and the receding planes will have much more importance and will not diminish in size so rapidly, while the amount of subject included in the background will of course be considerably less, and generally contribute to a more pictorial effect. With the lens we have described, the size of the image will correspond to that given by an ordinary lens of 15 inches focal length, and as the positive element works at an initial intensity of f/3, and its image has been subjected to a magnification of three times, the combination will work at an intensity of f/9, which is sufficiently rapid for the ordinary requirements of "instantaneous" hand-camera work. Many useful developments in this branch of photography suggest themselves. Fig. 61 represents a hand-camera in which the positive element is a stigmatic portrait lens of 6 inches focal length combined with a negative element of 3 inches focal length.
Telephotographic Hand-camera, which may be used with positive lens alone, by placing dark slide in the opening nearest the lens, or two degrees of magnification may be obtained when using the telephoto attachment by placing the slide in one of the other openings.
The box of the camera admits of a separation of 9 inches between the negative lens and the screen, thus giving a magnification of four times to the image produced by the positive lens alone. We thus have an image equal in size to that produced by an ordinary lens of 24 inches focal length which works at an intensity of/16. A camera of this kind is a veritable "detective" camera, and is valuable for rendering large images on small plates in studies of animal and bird life, ships at sea, distant mountain scenery, etc, and generally in all cases where the rendering of distance is inadequate with the ordinary hand-camera.