The telephoto is of importance not solely for record and pictorial landscape photography, but is also extremely useful for portraiture, giving large heads with the camera placed at a very great distance from the sitter, thus avoiding the distortion, amounting almost to caricature, which results from too close a viewpoint.
For the professional having at his disposal a. triple-body camera, the telephoto lens of vari able separation can be very simply rigged Up without special mounting. The convergent sys tem (which may be a Petzval or other type of portrait lens, or anachromatic symmetrical lens, according as a semi- or completely anachromatic lens is desired ; the first-named gives a better defined image, more agreeable to the taste of the average customer) is mounted on the front body ; the divergent lens will be attached to the middle body, so that its centre is on the optical axis of the convergent lens. In this case a plano-concave lens should be chosen, placed with its concave surface towards the front lens, the focal length of which should be almost half that of the convergent system and the diameter about one-third its own focal length. With a portrait lens of 18 in. focal length at 33 to 5o ft. from the sitter, portraits of 91X 71 in. size are obtained of very agreeable perspective, where the arrangement of the hands is easy and good " drawing " is possible (C. Puyo).
112. Sets of Lenses. We have already seen that convertible unsymmetrical lenses give the photographer three different foci, according as the complete lens or one of the components is used. By substituting for one or the other of these components other analogous combinations which can also be used separately, the number of foci available is increased, and thus a set or casket of lenses is formed. The total number of different focal lengths which can be got with a given number of components is shown in the following table— Number of component lenses . 2 3 4 5 Number of different focal lengths obtainable . . 3 6 so 55 It should, however, be pointed out that not all the possible combinations are, optically speaking, useful or possible. In fact, sets are generally limited to three or four components, and then rarely give more than five to eight combinations.
Lens sets are used chiefly by commercial photographers, who are often called upon to take a great variety of subjects out of doors under conditions which are often difficult as regards choice of viewpoint. It is rare for all the combinations to give images of a quality equal to that given by a specially designed lens. The combination of two lenses of very different focal lengths in relative positions which are not all equally favourable in fact hardly allows complete elimination of distortion, at least at the extreme limits of the field.
113. Different Types of Lens Mounts. Several types of mounts are employed for photographic lenses, according to the use to which they are to be put.
For cameras which are always used with a tripod (studio and field cameras), where there is no limit to the projection of the mount, a normal mount (Fig. 87), the simplest type of all, is usually fitted, the flange being attached' to the rear of the lens mount, giving free access to the diaphragm. On hand cameras, where bulk has to be reduced, a sunk mount is often prefer able, the flange being on the front part of the mount, which is thus sunk inside the camera in order to make use of the space corresponding to the thickness of the bellows (when closed). The diaphragm is then operated from the front by a cylindrical lining between the exterior mount and the actual body of the lens.
When the camera is not fitted with any adjust ment for focus a helicoidal focussing mount is employed, with the flange in front and com prising, beside the tube actuating the iris, three concentric tubes. Two studs fixed on opposite sides of the inner tube engage in helical slots in the intermediate tube and in slots in the outer tube parallel to the axis. A rotation of the intermediate tube by means of a lever working over a scale of distances causes the lens to move forward or backward without Finally, for use with between-lens shutters (§ 138), the mount is reduced to a pair of cells 2 in which the lenses are bezelled or held in with clamp rings, these cells being screwed into the female screws provided at either end of the