Page:EB1911 - Volume 21.djvu/539

with those of ordinary intensity the exposures are not unduly prolonged, and good definitions can be obtained over an extended field.

The optical principle on which these combinations are based is very simple, and will be understood from fig. 56. It depends mainly on the fact that in order that a real image may be thrown on the screen of an object AB, the rays proceeding from it, which pass through the positive system L₁, must come to a focus at a point f within the secondary focus f ″ of the negative system L₂. Falling within this limit, they will be intercept by L₂ and made less convergent, so that instead of coming to a focus at f, they will continue to converge till they reach the screen at f ″, and will there form a proportionally larger image a′b′ of AB than the image ab given by the positive lens alone at f; just as stated in Kepler’s problem. Moreover, this image a′b′ will be of the same size as if it had been produced directly by a positive lens L₃ with a focal length equal to lf ′ ″, and this distance is the equivalent focal length of the entire system. It can be found from the formula F＝f₁f₂/d, where f₁ and f₂; are the focal lengths of L₁ and L₂ respectively, and d=f₁+f₂−s being the distance between the lenses. In many instruments of the kind a scale showing the value of d is engraved on the mount. If the rays from AB come to a focus in front of L₂, on it, or beyond f ″, no real image can be projected on the screen. There is therefore a certain limit, which is greater in proportion to the length of focus of the negative system, within which the focus of the positive system L₁ may fall and produce a series of well-defined images on the screen, which can be varied in size by altering the amount of separation of the two systems of lenses within the above limit, and the distance of the screen from LQ. Every change in the position of the screen will involve a corresponding adjustment of the lenses. The greater the extension of the camera and the closer the lenses, the greater the size of the image and vice versa. The camera extension for a given magnification can be found by multiplying the focal length of the negative system by the number of magnifications, less one. The magnification produced by a given camera extension is found by dividing the latter by the focal length of the negative system, and adding one.

In its usual form (fig. 57) the telephoto graphic combination consists of a quick-acting portrait lens, or an an astigmatic doublet of

Fig. 57.—T. R. Dallmeyer’s Compound Telephotographic Lens.

large aperture and relative intensity of suitable focal length, fitted at one end of a tube, in which slides a smaller tube carrying a properly corrected negative system, which may vary in focus, but must be of shorter focus than the positive (usually about half); the shorter the focus the greater the magnifying power for a given extension of camera The amount of separation of the lenses is limited on the one hand by the position of the focus of the positive system, and on the other by the focus of the negative system, as explained above, and can be adjusted within these limits by a rack and pinion. The tubes are adjusted so that when closed up the two foci may coincide, or nearly so, and d=0, or its minimum value; and when opened to their fullest extent the focus of the positive may fall upon the negative system, or so that d may not exceed the focal length of the negative system within these limits the focal length of the combination will be positive; and a real image formed on the screen. Several forms of them have been brought out by various makes, some, as Zeiss’s, with a special positive lens, others for use with anastigmats and other lenses of large apertures. The negative lenses are also made of various powers.

Messrs Dallmeyer’s “Adon” (1902) is a telephoto graphic lens, for use with hand cameras, composed of two achromatic combinations adjusted for parallel rays, a front positive lens 4§ in. focal length, and a back negative lens of 21- in. ocus. These are mounted to permit of great variation in the separation, so that when the “Adon” is fixed on the front of a suitable lens, near or distant objects may be taken on an enlarged scale without altering the focus of the camera, or the enlargement can be varied with further extension of the camera Used alone it is a complete telephoto lens of moderate magnifying power, and will cover plates 15 in. ✕ 12 in. In 1903 a special form, the “Junior Adon,” was made in three kinds for use with kodaks and similar folding hand cameras, single and double extension, giving a fixed degree of magnification without loss of rapidity, while focusing can be effected by scale. It is intended to replace the front lens of an R R. or an astigmatic lens and cannot be used independently. Messrs Busch’s “Bis-Telar,” f /9 (1905), is another compact fixed focus telephoto lens, specially for use with hand cameras. It is a complete lens in itself, requiring no attachments and can be fitted to a central shutter. It is made in three sizes magnifying from two to three times. An improved form of this lens (1908), working at the large aperture of f /7 is similar to an old form of “Dialytic” lens worked out by J. Petzval, having a positive front and negative back meniscus,

Fig. 58.—“Bis-Telar”

with their concave surfaces facing inwards (fig. 58). As in the old “Orthoscopic” and lenses of that type there is some outward distortion, but it is very slight. These lenses are made in five sizes with foci from 8 to 22 in., requiring camera extensions from 41/2 in. to 111/2 in. They magnify about twice. According to K. Martin, a telephoto-combination of the Bis-Telar type can be used in a reversed position as a projecting lens for the lantern.

Captain Owen Wheeler proposed in 1907 a hi h-power telephoto arrangement, made by Messrs Staley, in which the negative attachment consists of three negative lenses, any single one of which can be used separately, giving magnifications of about 6, 9, and 13 diameters with a camera extension of 14 in. By combining the three a magnification of 30 diameters is attainable with the same short extension, which is a great advantage in many ways. In 1908 Messrs Zeiss issued their “Special Tele-objectives” in two sizes Working at f /10, the larger with an aperture of 3⋅14 in. and 32 in. focal length fitted in a special tele-camera for plates 9 ✕ 12 cm. with a monocular field glass magnify in four times as finder. The smaller one, with 18 in. focus, is adapted for hand cameras with 6 in. bellows extension. They consist of specially corrected positive and negative combination with a definite focal length and requiring a definite camera extension, and are specially suitable for balloon photography, instantaneous portraiture, &c. The theory, construction and use of telephoto lenses has been fully described by T. R. Dallmeyer in his Telephotography.

7. Anachromatic Lenses.—For large portraiture a certain amount of softness and diffusion of the image has long been recognized by artists as desirable, and in 1895 the “Dallmeyer-Bergheim Lens” was constructed with this special object. It is composed of a single uncorrected positive meniscus front lens, with a diaphragm in front of it, and an uncorrected negative meniscus back lens, and in the larger sizes it has great range of focal length on the telephoto graphic principle. The spherical and chromatic aberration produced by the uncorrected single lenses gives the diffusion of focus which produces the peculiarly soft and delicate effect aimed at. It is most useful for large heads and life-size studies, the great depth of focus conducing to uniformity of definition. There is no distortion, and by stopping down to about one-third perfect definition can be obtained. It works with great brilliancy, both elements being single glasses. It was the first of the anachromatic portrait lenses. Since 1903 Messrs C. Puyo and L. de Pulligny have been experimenting with various combinations of uncorrected lenses for producing the same effect in (Portrait and landscape photography by the diffusion of focus pro uced by chromatic aberration, and suitable lenses of this kind have recently been brought out in Paris as Les Objectifs d’artiste. In their construction the principal points to be considered are spherical aberration, to be minimized in the form and arrangement of the lenses selected; distortion, corrected by using a symmetrical system; astigmatism, avoided by using combinations of low power. The lenses used by Puyo have been: (1) a plano-convex crown with convex side in front at f /8 or f /9, or even f /5; for heads; (2) a simple thin concavo-convex meniscus, with concave side in front, is better and suitable for full lengths at f /10; (3) a symmetrical system formed of two similar crown menisci, concave sides inwards, is generally useful when worked at f /10, or even f /5. Arrangements are made in mounting these lenses for automatically making the necessary correction for colour. Another form is the “Adjustable Landscape Lens,” formed of an anterior plano-convex crown, 3 cm. diameter, and a posterior plano-concave crown, each of 10 cm. focus, and the same radii of curvature. In contact they have an infinite focus, but when slightly separated any focus can be obtained up to about 10 cm. In such a te photographic system, properly stopped down, an astigmatism, flatness of field, and rectilinearity are secured over a fairly large field. These lenses are fully described in Les Objectifs d’artiste, by L. de Pulligny and C. Puyo (Paris, 1906), and various forms, portrait and landscape, have been made by Messrs Hermagis, Turillon & Morin (see Fabre, T. E. P. Suppl. D. 101).

Diaphragm Apertures.—In order to regulate the intensity of the illumination by the lens, to enlarge its field, and, in the case of the older forms of objectives, to extend the area of good marginal definition, diaphragms are used, usually with circular apertures. They are made in different ways: (1) as single metal lates, fitting into a slot in the lens tube (Waterhouse diaphragms); (2) Rotatory: a single plate revolving on a central axis and pierced with apertures cut to fit centric ally in the opening of the lens; (3) Iris: a form of diaphragm now very generally used, and very convenient, because it can be easily adjusted as required for intermediate apertures. As a rule they are placed at the optical centre between the elements of a compound lens or in front of a single one.

In order to provide a uniform system of diaphragm apertures,