The ideal apparatus for stereoscopic photo graphy consists of a camera furnished with a pair of carefully matched achromatic and recti linear lenses of 5 in. to 6 in. focal length duly corrected for spherical aberration. The distance between two such lenses should be adjustable, so that for near subjects their separation may be reduced to 2 in., or for remote subjects increased to 3 in. or 31 in. The mechanism controlling the opening and closing of the two shutter apertures should be such as to ensure absolutely synchronous working. Lenses of a shorter focal length than 5 in. are unsuitable for stereoscopic work, as they appear to give an exaggerated perspective which is especially noticeable in architectural subjects. Again, too great a separation of the lenses on a stereo scopic camera induces abnormal relief in the stereoscope, besides making it difficult for the two pictures to be seen blended together at all planes in the composition.
The actual practice of stereoscopic photo graphy, once the principle has been grasped, is simple. Exposure and development are as usual, taking great care, however, when the two images are on separate plates, to obtain results as uniform as possible. Hand-work on the negative is not desirable, and, indeed, will militate against success. It is only in the
transposition of the prints that there is any difficulty. It has already been shown that by some methods of stereoscopic photography, transposition of the prints is not necessary. Again, it is possible to cut the negative and to transpose the halves in a special printing frame so that the print obtained is suitable for use without alteration ; but in the case of photo graphs taken in two-lens cameras, and when the negative is not cut before printing, the following procedure will be necessary ; Lay the print face downwards, and lightly mark the edges of the print so that the sequence of the images can be recognised at a later stage. Assume that, as the print lies face down, the image on the left is No. 1, and that on the right No. 2. Carefully trim, leaving on the right of the right-hand print + in. more of the picture than appears on the left-hand print ; in the same way leave on the left of the left-hand print + in. more of the picture than appears on the right-hand print. Next sever the two images and mount the prints, being careful to place them about + in. apart. Looking at the face side of the prints, No. I will be on the left and No. z on the right.