TAKING STEREOSCOPIC NEGATIVES 837. Usual Stereoscopic Sizes. Stereoscopic apparatus, consisting of two coupled cameras, usually form the two images of the pair on a single plate or film. 2 The usual size of the first stereoscopic transparencies was 85 x 170 nun. (approximately 31 x 61- in.), the two joined images each measuring about 75 X 75 mm. (3 x 3 in.), with lateral margins of to mm. (*.-: in.), corresponding to the position of the binding and title. These transparencies were printed from negatives taken on plates 8 x r6 cm. (approxi mately 31 x 6 in.) or 9 X i8 cm. (3), X 7 in.).
The International Congress of Photography held at Brussels in 1891 recommended the same size, 85 X 170 mm., with single images 66 X 70 mm. (2 X 2 in.), separated by a band of width 4 min. (1 in.), leaving thus a distance of 70 inm. (21 in.) between the principal points,. This recommendation was never used in prac tice, and was changed by the Congress held at Brussels in 191o, where it was decided to allow " all sizes which can be held in a frame formed by two squares of side 8o mm. (3;', in.), placed side by side, it being assumed that homologous points of an object at infinity must not, under any circumstances, be more than So rnrn (31 in.) apart, and that a smaller distance is preferable." However, negatives and transparencies of the same size are now usually used, the single images being very nearly square. The stereoscopic camera cannot be turned through a right angle for taking " vertical " and " horizontal " pic tures ; square images are therefore chosen to give the best results in both cases. Vertical and horizontal pictures may be obtained by masking the square negatives or by making suitable enlargements. The sizes most frequently used in modern stereoscopic cameras are given in the following table- There are also excellent cameras of 9 X 12 cm, and io X 15 cm., generally fitted with three lenses, two for stereoscopic work and the other of longer focal length for a single image on the whole of the plate.
838. Stereoscopic Cameras Having Two Lenses. Stereoscopic cameras made to take the two stereoscopic photographs simultaneously are usually hand cameras of as varied designs as ordinary cameras (Chap. XIV), and are fitted with the same accessories.' Since stereoscopes are not fitted with means for decentring the eyepieces, if one wishes the reconstructed object to be geometrically similar to the object itself the camera lenses used in making the negatives must never be decentred. It is to reduce the need of such decentring that lenses of very short focal length are frequently used. As, however, the negatives of a stereo scopic pair are often used separately for printing, enlarging, or projecting, that is to say, when decentring is of some importance, manufacturers make a compromise by fitting their cameras with a vertical movement of small range. Some makers of simple cameras, who, on account of price, cannot use wide angle lenses, consider that, for the majority of amateurs, conditions of geometric similarity are secondary to the advantages of decentring. Whereas, in ordinary photography, it is rarely necessary to ensure that the principal point corresponds to the horizon, their cameras are fitted with the lenses raised above the centre ; if the photographer wishes to work from a high point he can use the camera upside down, thus getting a lowering of the lenses. The stereoscope corresponding to such a camera must obviously have its lenses decentred by the same amount.
The two lenses of a stereoscopic camera must be identical, so that they produce images in focus of the same dimensions and luminous intensity at the same distance.
Trouble would inevitably occur if the various adjustments were made on each lens separately : the focussing mechanism and the iris diaphragm are coupled by means of rods, except in the particular case when the separation of the lenses can be controlled.