Britannica Sources used in this answer

How do stereoscopes use parallax to create a three-dimensional image?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram illustrating how astronomers calculate the distance to a star using parallax: Earth is shown in two positions on opposite sides of its orbit around the Sun, with lines extending to a nearby star X and further to distant background stars. The apparent shift in star X's position, measured as angle A, is used to determine its distance, with the baseline labeled as 186,000,000 miles. — Parallax Calculating stellar distances.

Stereoscopes create a three-dimensional image by exploiting the principles of binocular vision and binocular parallax.12

Because human eyes are separated by a small distance, each eye perceives an object from a slightly different angle. This difference in the retinal images is known as binocular disparity or binocular parallax. The brain interprets this disparity to perceive depth and distance; the greater the parallax difference, the closer an object is perceived to be.2

A stereoscope simulates this natural process using a pair of two-dimensional images, called a stereopair, which depict the same scene from slightly different angles corresponding to the views of the left and right eyes. The stereoscope acts as an optical device that presents the left-eye image exclusively to the left eye and the right-eye image to the right eye. By forcing the eyes to view these disparate images simultaneously, the device allows the brain to fuse them into a single, three-dimensional perception of space.13