What constitutes a stereo image? The normal, single photographic image is called a “monocular” image since it stands by itself. The term “stereo” means two and is considered a “binocular” set of images. The stereo camera takes these two images simultaneously using two lenses. The images are spaced roughly the same distance as our “interocular” distance, the spacing between our eyes. Using a “Holmes” style viewer (such as the antique stereoscope) or small “lorgnette” glasses to view the stereo pairs brings the views together where we are able to see 3D depth.
The compositional approach is different between a single image photo and a stereo photo. In single image photography, the photographer tries to avoid anything in front of the object such as a tree in front of a building. An angle will be chosen to photograph the building directly, without obstruction. This can be for artistry or documentary purposes. In stereo photography, the stereographer looks for something in the foreground that compliments the main object in the background so there is a visual, spatial cue when the images are combined to show the depth in the image. This can be a rock, tree limb or any object. In some images, the main object will be in the foreground and the spatial cue object is framed behind. For maximum depth perception, it is best if the foreground subject is fairly close to the camera with the background object farther away. The realistic perception of depth is called “immersion.” The more the viewer is immersed in the image, the more real the perception.