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Technical Notes
LENSES WITH VARIABLE FOCAL LENGTH
The problem of constructing lenses with variable focal lengths has for some time been the subject of study by the makers of optical material. The principal object aimed at with such lenses is twofold, 1) the possibility of obtaining pictures of various sizes at the same distance without changing the lens ; 2) the possibility of obtaining a longitudinal carriage (bringing the subject closer or making it further away) without moving the camera itself.
Two such types of lenses have been recently constructed ; one by Bell and Howell and the other by Otto Durholz of Paterson. The " Varo " lens of Bell & Howell, which is really a special device, is enclosed in a rectangular box almost as large as the camera itself, and is applied to the anterior part of it. The focal length of the optical system may be varied continuously from 40 to 120 mm, and the variation can be made during the actual photographing. Naturally, during such operation the various parts of the lens move in such a manner that the subject is continually kept under its eye. The dimensions of the pictures vary in the proportion of one to three, that is naturally when the whole field of variation of focal length is used. The maximum relative apertures do not, however, remain constant for all the focal lengths. At between 40 and 50 mm, such maximum aperture is F : 3,5. For the intermediate zones, up to 85 mm, the aperture is diminished to F : 4,5, while for the other variations from 40 to 120 mm, the maximum apertures are from F : 5,6 to F : 8.
With regard to the focussing, it must be
observed that this does not take place, as with ordinary lenses, by means of varying the distance of the plane of the image, but by a focussing system that is calculated from 1 50 feet to infinity. This really means that with the lens in question the position is less favourable than with ordinary lenses, for the hyperfocal distance for 40 mm and F : 3,5 is about 14 metres ; for 50 mm and F : 3,5 it is about 21 metres ; for 85 mm and F : 4,5 it is about 48 metres, and for 1 20 mm and F : 5,6, it is 77 metres. When it is necessary to photograph at a distance, additional lenses must be employed, which are attached to the front section of the device.
The variation of the focal length is obtained by means of a handle placed at the side.which gives a synchronized movement of the various parts of the device, each part moving according to its own laws.
The solution of the problem offered by the firm of Durholz seems simpler. The optical system has in this case the appearance of a telephoto lens, easily mountable in the place of one of the normal lenses, and with a revolver-shaped head. Its weight is about 2200 kilograms.
According to the instructions issued, the focussing is very easy, and can be made from a distance of one metre without any need of adding additional lenses . The focal length may be changed continuously between 40 and 160 mm. In fact, when it is necessary to cover the reduced superficies of the sound film, the minimum focal length can be reduced to 37 mm. In this case, as in the foregoing, the maximum aperture varies with the focal length. Precise details are lacking.
5 — let ingl.