How Do Humans Judge Distance With Only One Eye?

Depth perception is the ability to determine how far away an object is in relation to the viewer. Typically, the mind accomplishes this by triangulating the object's position using your eyes as two corners of the triangle and the object as the third. This process is known as stereoscopic vision. For people with only one eye, the mind needs to use other tricks.

Accommodation

• When viewing objects within four feet of the eye, the human eye bulges in or out to focus on the object. This focusing action sends feedback to the brain. Your brain can tell how far away an object is based on the action performed to bring the object into focus.

Relative Size

• Your mind has an innate sense of size. If it sees an object that looks like an incredibly tiny chair, it reasons that it is seeing a chair from considerable distance. Even if the mind doesn't recognize an object, it can typically recognize another nearby object to compare size,

Head Movement

• By moving their head up or down a certain amount, a person can create a phenomenon similar to the triangulation created by having two eyes. Many animals with eyes on opposite sides of their heads rely on this trick and humans can as well.