Human vision uses several cues to determine relative depths in a perceived scene. One of these cues is known as Stereopsis.
Stereopsis (from ‘stereo’ meaning “solid” or “three-dimensional”, and ‘opsis’ meaning view or sight) is the process in visual perception leading to the perception of depth from the two slightly different projections of the world onto the retinas of the two eyes.
The differences in the two retinal images are called horizontal disparity, retinal disparity, or binocular disparity.. The differences arise from the eyes’ different positions in the head.
Traditional stereoscopic photography consists of creating a 3-D illusion starting from a pair of 2-D images. The easiest way to enhance depth perception in the brain is to provide the eyes of the viewer with two different images, representing two perspectives of the same object, with a minor deviation exactly equal to the perspectives that both eyes naturally receive in binocular vision.
If eyestrain and distortion are to be avoided, each of the two 2-D images preferably should be presented to each eye of the viewer so that any object at infinite distance seen by the viewer should be perceived by that eye while it is oriented straight ahead, the viewer’s eyes being neither crossed nor diverging. When the picture contains no object at infinite distance, such as a horizon or a cloud, the pictures should be spaced correspondingly closer together.
The principal advantages of side-by-side viewers is that there is no diminution of brightness so images may be presented at very high resolution and in full spectrum color. The ghosting associated with polarized projection or when colour filtering is used is totally eliminated. The images are discretely presented to the eyes and visual centre of the brain, with no co-mingling of the views.
A popular technique for viewing 3-D photographs was the “American stereoscope” invented by the American author Oliver Wendell Holmes, Sr. (1809–1894). The stereoscope is a hand-held viewing device Two photographs of the same scene are taken a few inches apart, and printed ‘side by side’ on photographic paper to make what became known as a ‘Holmes Card’.
The Holmes Card is then inserted into the viewer which has special lenses that create the illusion of 3D. This is the technique that was adopted by David Buik for his 3D pictures. The Local History department of Dundee City Council have a number of Holmes Cards in their collection. The classical Holmes card is 3.5 x 7 inches (89 x 178mm). Each picture is 3 inches square (76.5mm). Actual measurements on antique Holmes cards shows not all followed this rule. For example, “Rose Stereographs” of New Zealand were sometimes 87.5 x 73mm on a 4 x 7 inch card (101 x 178mm).