There are different types of depth perception, known as depth cues. These cues are classified into binocular (both eyes), monocular (one eye), and inferred (combined binocular and monocular cues). All require input from the eye(s) to the brain in order to perceive depth, but it depends on which cue a person has to determine how well they perceive distances and sizes.

The term stereopsis means that a person sees clearly with two good eyes, and he/she sees images with stereoscopic vision. Someone who only sees with one eye lacks this tool and must rely on other cues to determine depth. When someone uses both eyes to focus on the same object, they converge. The convergence then stretches the extraocular muscles, and kinesthetic sensations from the extraocular muscles help with depth/distance perception. Other binocular cues include:

Retinal disparity – Disparity means different. So retinal disparity simply means that each eye receives a slightly different image due to the angle from which each eye is viewing an object.

Fusion – When the brain brings the retinal images from the two eyes to form one object, its called fusion. Fusion takes place when the objects appear the same.
Monocular cues allow a person to perceive depth and sizes of objects with one eye. Relative size is when two objects are known to be the same size, but their actual size is unknown. Relative size then allows someone to estimate and perceive relative depth of the two objects. Other monocular cues include:

Interposition – Interposition cues occur when there is an overlapping of objects

Linear perspective – When objects of known distance appear smaller and smaller, it’s interpreted as these objects being further away.

Aerial perspective – The relative color and contrast of objects gives us clues to their distance. When scattering light blurs the outlines of objects, the object is perceived as distant.

Light and shade – Shadows and highlights can provide information about an object’s depth and dimensions.
Monocular movement parallax – When our heads move side to side, objects at different distances move at different speeds, or relative velocity. Closer objects move in the opposite directions of the head movement, and farther objects move with our heads.

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