If the image given by the complete system is magnified In times (ratio of lengths) relatively to that which the convergent system alone would have given, the distance E of the divergent lens from the magnified image is E — x) If, in the case of a telephoto, where f1/f2 = s, the distances 0 and I of the object and image respectively from the telephoto and the distances o and i for an ordinary lens of the same focal length giving an image of the same size, are calculated, it is found that 0=0 + [F(s —I) +11] I=i — [F(1 — Ils) +12] In one or other of these forms the telephoto requires careful handling on account of the very faint image, making focussing difficult when large magnifications are attempted. The use of an optical system with variable separation requires in fact that each of the components should be separately corrected, which is only possible with small apertures. This type of lens must thus be considered as suitable only for special purposes. There is no doubt, how ever, that it can render very appreciable service. The use, in place of it, of enlargement of part of a negative taken with an ordinary lens is not to be thought of, for at the same time the granular structure of the image is enlarged, which limits the degree of enlargement to not more than ten times as a rule. 1 no. Fixed-focus Telephotos. Very great The ratio s being always greater than (or at least equal to) unity, it is seen that, for a photo graph on the same scale, the distance of the object from the telephoto is always greater than it would be with an ordinary lens of the same focal length, and the more so the smaller the focal length of the amplifying negative lens compared with that of the convergent system. Conversely, the distance from the image to the telephoto is always less than it would be with a lens of the usual construction.
The first telephoto lenses used any ordinary photographic lens as the convergent system, and as amplifier a more or less complex divergent system. The two components were mounted so that the separation could be altered by a rack and pinion, the optical interval (3 of the above formulae being marked on a scale on the outer tube ; also, usually, the resulting magnification and the corresponding distance of the image.
The first simplification took place in 1896 by the construction of a complete system shown in Fig. 84, the divergent system being shown in full lines in the position for a focal length of 4 in., whilst in the position shown by the dotted lines the focal length is about 12 in.
improvement, at least in the construction of tele photo lenses for general use, was made when K. Martin (1905) abandoned the variable mag nification and made a lens which, in view of its employment on hand cameras, would be more correctly described as a long-focus lens for short-extension cameras. The possibility of compensating the aberrations of each of the components by aberrations of opposite sign in the other, and the removal of the difficulty of centring which arises when two sliding tubes are employed (which must necessarily have a little play) enabled him to obtain an aperture of F/9 covering about 35° and giving an image of a quality comparable with that of a good rectilinear. The lens had the advantage that
for a focal length of 9-1 in. (covering almost the whole of a 4 X 31 in. plate), the distance from the vertex of the back lens to the plate (prac tically the same as the distance from the camera front to the plate) was only 54-- in., that is, only 58 per cent of the focal length. The aperture of these lenses was extended afterwards to F/7 or F/7-7 (by the same designer), with a slightly smaller field, the focal length for the 4 X 31 in. plate being increased to roi in., without increase of camera extension.
This very important reaction against the use, on amateur cameras, of lenses of small focal length which, though useful for record work, are opposed to all aesthetic rules, has been supported by the greater number of English and German opticians, who have all kept to the same ratio, which is a very suitable one, between the focal length and the camera " extension," while striving to increase the aperture and the quality of the image. In England, where this type of lens seems to have reached very great perfection, the aperture has been increased, first to F/6-8 and then to F/3-5, with a field of almost 35°. Fig. 85 shows a lens of F/4•8 aperture by H. W. Lee (1922). These different lenses have been made up to now only for use on hand cameras of size not exceeding 7 x 5 in. The most recent types give an image comparable with that given by the best modern lenses of normal construction.' Anachromatic Telephotos. The use of a telephoto lens of low but variable magnification is of importance to the landscape photographer, who, having chosen his viewpoint, must make his image cover his plate. On the other hand, the distribution of sharpness between the suc cessive planes, without exaggerated softness in the distance, leads him to the use of a smaller relative aperture as the focal length is increased. In fact, it may be said (C. Puyo) that the abso lute diameter of the greatest effective aperture of service in artistic landscape work should be in the neighbourhood of o-8 in. For 18 X 24 cm. (91 x 7 in.) plates, the focal lengths mostly used lie between 16 in. and 25 in. The anachromatic telephoto for which C. Puyo and L. de Pulligny gave data in 1906, which they called adjustable landscape (Fig. 86), is very suitable for this purpose. It is constructed of two lenses of 0-8 to 1.2 in. diameter, one convergent and the other divergent, of the same focal length, e.g. about 4 in. for 9 x 7 in. plates, or 3 in. for 7 X 5 in. plates. These are mounted so that the separation can be varied without exceeding a quarter of their common focal length. A diaphragm of 0-8 in. aperture (a little less for lenses of very short focal length) is placed in front of the convergent lens, and a variable diaphragm can usefully be placed behind the negative amplifying lens for varying the degree of softness. Such a combination, used over a small field, can be sufficiently corrected for astigmatism, and have an almost flat field without too much spherical aberration.