Stereopsis refers to our ability to appreciate depth, or to distinguish the physical distance between objects. Without depth perception it is difficult to pour water into a glass from a pitcher, appreciate a 3D movie and parallel park a car. An Example: The Stereo Fly Test from Stereo Optical is used to evaluate both gross and fine stereo vision.
A person taking a Stereo Test with polarized glasses will report that the objects look closer to them if they have normal binocular vision.
Question: What do you suppose happens if you turn the test book upside down?
Answer: The same test objects look farther away. Try it!