What is Depth Perception? Binocular and Monocular Cues For it

Depth Perception

Depth perception is the ability to perceive the world in three dimensions (depth and distance). It is about how we perceive the distance and the depth of things.

Psychologists have been puzzled by the question of how we can perceive depth or distance. The surface of the retina is two-dimensional. It has up and down, and a left and a right, but no depth. Even then we can perceive a three-dimensional (3D) world very easily. The eye and brain accomplish this by using two main types of cues: binocular and monocular cues.

Binocular Cues For Depth Perception

Binocular cues require visual input integrated from the two eyes for depth perception. The stimuli can only be perceived clearly by using both eyes. In other words, binocular cues are the ability to perceive the world in 3D by using both eyes. Convergence and retinal disparity are the two binocular cues:


When we look at something our eye muscles move the eyeball to allow us the view. It is done by eye muscle adjustment. These muscles also provide feedback for judging distance. When we look at the objects nearby, the eyes rotate inward toward a center point. In convergence, the eyes turn inward, when we focus on nearby objects than on distant ones. Convergence cue is more kinesthetic than visual because it is produced by muscle movement in the eyes.

Retinal Disparity

Because our eyes are about 2*1/2 inches apart our retina receives slightly different pictures of the same object or situation. The right eye sees more of the right side of an object, the left eye sees a bit more of the left side. Vision is clear if involves both eyes. This difference in the two views is referred to as retinal disparity, the closer the object is to the viewer the greater the disparity.

Monocular Cues For Depth Perception

Monocular cues can be defined as the ability to see the depth of the world by using one eye. Monocular cues are seen by one eye alone. There are different monocular cues to perceive depth and distance perception. They are:

bird flying over Taj Mahal
Taj Mahal


When one object blocks the path of another, the blocked object is perceived as more distant. In the figure, the trees blocking the sidewalk and the front of the building, elevation.

Linear Perspective

The decrease in size and in the separation of the object as they become more distant is known as linear perspective. Railroad, telephone poles, trees, and other objects decrease in size and railroad tracks coverage, as they are farther away from us. The figure depicts the convergence of the lines of the walkways surrounding the water as it approaches the Taj Mahal.

Aerial Perspective

Since light scatters as it passes through space and especially through moist or polluted air or the haziness, objects at greater distances appear fussier than those nearby. A mountain, a building, or any object which is nearby is seen in greater detail than the more distant objects because of the atmospheric condition, is the aerial perspective. In the figure, the lack of details of the bird in the distance shows the aerial perspective.

Retinal Image

The size of the retinal image is normally larger for nearby objects than for distant ones. We perceive an object as farther away when its retinal image is smaller, but we must be acquainted with the actual size of the object to make this distinction. The middle part of the sideways in the above figure is an example retinal image.

Texture Gradient

The texture of a surface becomes smoother with increasing distance. A grained wood, the raw stone appears rough at close range, and finer at greater distances. Likewise, the meadow at the other side of the fence appears much greener than one’s own side. The relative clarity of the breaks in the sideways closer to the camera illustrates the texture gradient in the figure.


Brighter objects appear close than duller to create the illusion of depth perception. Artists use this concept to draw sizes and shapes of objects, faces, etc. even in plane surfaces to create different depths and distances. The indentation of the arches toward the top of the building signifies shading in the figure.

Movement Cues

Movement cues act in different ways. When we are standing still and two objects are traveling at the same speed, the object which seems to move by using rapidly is closer than the one which moves by slowly.

If we ourselves are moving, objects nearby seem to move past us in the opposite direction to that in which we are traveling, while distant objects appear to move in the same direction.


The shape of the lenses of the eye must change to focus the visual image on the retina from stimuli that are different distances from the eye. This process is called accommodation. This is mediated by ciliary muscles attached to the eye mechanism. The shape of the lenses increases as the focuses on the objects close at hand, and it decreases when more distant objects are brought into focus. The kinesthetic receptors in the ciliary muscles provide the brain with information about the distance of the different objects.

All the depth perception cues described above are the psychological cue because they depend upon the characteristics of the visual image, while accommodation is is the physiological cue because it arises from the structure and movement of the eyes.

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