Grades 6–8 · Arts & Music
How can two flat pictures create a sense of depth?
A stereoscope can make a pair of flat photographs seem to contain depth. Its secret begins with two viewpoints. Our eyes are separated, so each normally sees the world from a slightly different position. Nearby features change position between those views more than distant features do.
In the diagramTwo viewpoints give different near positionsLeft-eye pictureRight-eye pictureFar triangle aligned · near block shiftedPositions exaggerated; a viewer sends one image to each eye.
The picture aligns a distant triangle in two invented views. The nearer block then sits at different positions relative to that triangle. This difference between corresponding image positions is called binocular disparity. Moving the block farther away reduces the relative difference. At the triangle’s distance, it has no relative shift in this simple model.
A stereoscope uses lenses or mirrors to send one member of a suitable picture pair to each eye. The visual system can combine their information and use the differences as a cue to depth. Simply printing the same picture twice does not supply those viewpoint differences. Binocular vision is only one depth cue, and people vary in how they perceive stereoscopic images. The explanatory shifts here are enlarged; these diagrams are not an eye test or a pair designed for viewing through a stereoscope.
About this explanation
Starting knowledge: Understands near versus far and that eyes occupy different positions.
Sources
- Science Museum Group — Whittington stereoscope, London, 1925–1940
- MIT Press / Foundations of Computer Vision — Stereo Vision