Stereoscopic Photography...the Science of Solid Viewing
The Stereoscope for Print Viewing
This is the nub of the whole system for its optics must present left and right wide angle print images to the viewer's respective eyes. These print images are wide angle or to be more exact dimensionally compressed. To see them correctly they must be presented wider than the viewer's normal gaze. That lot was very difficult to do and required much amusement with geometry to achieve it.
Originally to get started with this project an old prism stereoscope made in 1900 was used. Optically it was good but would only take small size square prints (80x80 mm). A hundred years ago good twin lens stereoscopic cameras and matching prism stereoscopes were common place. The left and right images were on the same film and contact printed as a pair. This system (in modern form) survives to this day because it is inexpensive and convenient.
The stereoscope that is used for this project is just a modern version of the 19th century mirror Wheatstone instrument. This mirror design is technologically updated for the specific purpose of viewing photographic prints. It uses four 45 ° first surface mirrors to direct the viewers sight down and out to two sets of enlarged prints set either side by side or one above the other. The viewer's left eye sees the left print and the viewer's right eye sees right print. Lenses are used to direct and relax the viewer's eyes to distant vision and slightly magnify the image. This instrument is large, bulky and expensive but delivers great pictures.
The Stereoscope with one print above the other is theoretically a better design but lighting and print exchange are difficult to arrange. Hence this 'one above the other' design works best with transparent back lit fixed pairs, though it can be used with prints as illustrated. This particular design cannot be adapted for eye individual spacing but must be set at the 'normal' 60 mm.
A stereoscope needs good lighting, print exchange, and optical flexibility. These requirements are achieved with the prints side by side design. In this the print pairs are mounted on Jacquard belts that can be advanced by turning a handle. The side by side design can be adapted to adjust for individual eye spacing and to some extent for eye focus irregularities.
The mirror system is easy to represent, for design purposes, by a simple geometric design model. This is fortunate for the dimensions, alignment of the mirrors and the focal length of the lenses are all critical specifications. The current instrument is ideal for 127 mm x 190mm prints which have to be commercially interpreted as 5 x 7 inch (127 mm x 178 mm). Were the prints to be viewed as standard 4 x 6 inch size (100 mm x 150 mm) the images would seem to fill the viewer's direct gaze; thus laterally compressing the images. The larger photo images over fill the viewer's direct gaze thus appearing to expand the images. Thus the viewer may concentrate on a chosen portion of the image, as we do with normal sight.
This print size legerdemain along with the optical design of the stereoscope compensates for the taking wide angle, exaggerated perspective and the wide camera spacing parallax exaggeration.
Operation of the Stereoscope
The side by side stereoscope geometry can be expressed in terms of an algorithm. Using this algorithm the design can be made optimum for print sizes up to 5 x 7 inch (127 mm x 178 mm).
The above picture shows the prototype table model stereoscope for side by side prints. The prints are held in alignment on a plate that can be flipped over to move the Jacquard belts along. Lighting is provided by full spectrum fluorescent light that normally stands against the back.
For use beyond the laboratory improvements would be needed: