The application of the Telephotographic lens for photographing distant objects has been that most generally employed. In this connection the Telephotographic lens does not differ in its performance from a lens of ordinary construction of the same focal length, but its advantage over the latter lies in the fact that we may produce very large images corresponding to those given by lenses of very great focal length without the necessity for the accompanying great camera extension which the latter involve. The fact that almost any positive system may be converted into a Telephotographic combination by combining it with a negative element whose function is simply to magnify the primary image has made the practice of Telephotography both wide-spread and popular.

It is preferable that the positive element composing the system should have at any rate an intensity not less than//8 for the reason given in Chapter VI (The Use And Effects Of The Diaphragm, And The Improved Perspective Rendering By The Telephotographic Lens)., viz., that the limit of magnification which may be given to the image produced by it is comparatively small, without introducing the deleterious effect of diffraction. This, as we have said, commences to assert itself when the intensity of the compound system is below f/71.

It is obvious, then, that the effective aperture of the positive element controls the limit of magnification, corresponding to a certain equivalent focal length, which must not be exceeded. The equivalent lens must not be more than 71 times the effective aperture of the positive system ; so that if very high magnification is desired, our positive element must be large in diameter. A positive lens of i-inch aperture must never be converted into a Telephotographic system in which an equivalent focal length of more than 6 ft. should be brought about; with a 2-inch positive lens, 12 ft, and so on.

The particular constructions which have been designed for the purpose include as positive elements: portrait lenses, any non-distorting doublet, such as a rapid rectilinear, the antiplanat, anastigmatic, stigmatic, etc. We will now shortly describe the various Telephoto-graphic constructions which are in use.

The author's original construction (1891) consisted of a single cemented positive element of high intensity combined with a single near and distant objects, unless some means be given to regulate the spherical correction. When this is not possible, it is obvious that the only means of attaining a high degree of definition is by use of the diaphragm, involving a loss in rapidity. In the author's combination, the system is corrected to give the finest definition on a moderately near object; but if it be employed for photographing a very distant object, a slight unscrewing of the back element of the portrait combination will adjust the spherical correction. The necessity for this is most apparent when a high power is employed. The stigmatic lens of intensity f/4 also has this means of adjustment, rendering it eminently suitable for the positive element of a Telephotographic system.

Fig. 62.

IV Telephotography For Distant Subjects 113

Stigmatic lens - converted into Telephotographic lens of moderate power by the addition of a negative lens of J its focal length, f1/f2 = m = 2.

cemented negative element, with a diaphragm placed between the combinations, The inherent defect of pincushion distortion led him to abandon this form of combination. The chief aim, when the Tele-photographic construction was first made, was to dwell on the astounding results produced by high magnification ; and, as already pointed out, if the ratio of the focal lengths of the elements composing the system, or m, is high, the distortion is so palpable as to render its employment highly undesirable. He therefore substituted either a portrait combination of high intensity, f/3, or any other non-distorting combination as positive element, of an intensity not less ihan f/8, combining it with a double combination negative element as illustrated in Fig. 62. This form of negative element was constructed to give the greatest freedom from distortion, and was found preferable to the single cemented (triple) combination formerly employed. By employing the portrait lens already referred to, in which the spherical correction is adjustable, the author was enabled to produce a Telephotographic system which is perfectly corrected or able to produce sharp images of either near or distant objects. When the positive element of the system is combined with a negative lens for the purpose of attaining high magnification, the spherical correction cannot be perfect for both

Fig. 62a.

IV Telephotography For Distant Subjects 114

Messrs. Steinheil of Munich have given considerable attention to various types of instrument in which the negative enlarging lens is employed. They have produced interesting telescopes of great compactness and yet possessing high power, the magnification being arrived at by the employment of a negative enlarging system which increases the focal length of the object glass without necessitating a long telescope tube. Messrs. Steinheil also construct telescopes for photographic purposes on the same principle, which are designed for high magnification, but to include only a very small angle. For ordinary Telephotographic work, they combine their Group Antiplanat as positive element with a triple cemented negative as shown in Fig. 62A.

Fig. 63.

IV Telephotography For Distant Subjects 115

Messrs. Zeiss of Jena construct two types of a Telephotographic lens. We have already referred to the form designed for portraiture illustrated in Fig. 57. They recommend their double anastigmat combined with a triple cemented negative lens for all cases in which distortion is inadmissible. This combination is illustrated in Fig. 63.