Binocular vision is vision using two eyes. Because the two eyes occupy different positions, each receives a slightly different perspective view of spatial reality. The human visual system combines information from these two views to support a single visual experience and to provide important information about depth, distance, position and three-dimensional form.
Binocular vision is an important component of human spatial perception, but it is not the sole means by which we perceive depth. Human vision combines binocular cues such as stereopsis and vergence with numerous monocular depth cues, including perspective, diminution of size, foreshortening, occlusion, motion parallax, texture, horizon relationships and atmospheric effects.
Understanding binocular vision therefore requires understanding its relationship with the wider systems of visual perspective, optical perspective and depth perception.
What Is Binocular Vision?
Binocular vision is the ability to use the two eyes together when viewing spatial reality. Each eye forms its own retinal image from a slightly different viewing position. The resulting images are similar, but their corresponding features can differ in apparent position, shape and viewing angle.
The visual system processes and combines these two streams of visual information. Differences between the two eye views can then provide useful information about the relative positions and depths of objects within the surrounding three-dimensional scene.
Binocular vision is therefore simultaneously an optical, geometrical, physiological and perceptual process.
Two Eyes, Two Perspective Views
The key geometrical fact behind binocular vision is that the two eyes view the world from two separate positions.
An object therefore normally subtends a slightly different direction and aspect at each eye. Nearby objects show greater differences between the two views, while the angular differences associated with more distant objects become progressively smaller.
Binocular vision can consequently be understood as a natural form of multi-view perspective: two closely separated views of the same spatial reality are available simultaneously to the visual system.
Binocular Fusion
Binocular fusion is the process through which visual information from the two eyes is combined into a coherent perceptual experience.
The two retinal images are not exact duplicates, because each eye has a different viewpoint. Nevertheless, corresponding visual information can normally be brought together so that we experience a unified scene rather than two unrelated images.
The Dictionary of Perspective distinguishes sensory fusion—the perceptual combination of the two retinal images—from the associated motor adjustments that help maintain binocular alignment.
Binocular Disparity and Binocular Parallax
The small differences between the views received by the two eyes are commonly described through binocular disparity or binocular parallax.
Because each eye sees an object from a slightly different viewpoint, corresponding features can occupy different relative positions within the two images. The visual system can use these differences as information about relative depth.
This is one of the principal geometrical foundations of stereoscopic vision.
What Is Stereopsis?
Stereopsis is the perception of depth arising from differences between the two eye views. It is one of the principal binocular depth mechanisms and is closely associated with binocular disparity.
Stereopsis can provide particularly precise information about relative depth at close range. This helps with tasks requiring fine spatial judgements, such as locating, reaching for or manipulating nearby objects.
Its effectiveness decreases as viewing distance increases because the differences between the two eye views become progressively smaller. Binocular stereopsis therefore contributes strongly to near spatial perception but is not the principal explanation for our perception of extensive depth across large or distant environments.
Binocular Convergence and Vergence
Binocular convergence, or vergence, provides another source of information about distance.
When a person fixates a nearby object, the two eyes rotate so that their visual directions meet at that object. As the fixation distance changes, the relative angle between the eyes changes accordingly.
The visual system can use information associated with this changing eye alignment as one contribution to distance judgement.
Vergence should be distinguished from stereopsis. Vergence concerns the relative directions of the eyes; stereopsis concerns differences between the two retinal images. Both can contribute to binocular depth perception.
The Binocular Field of View
The human visual field is formed from two overlapping monocular visual fields.
The total horizontal field visible with both eyes is approximately 200 degrees, while the central region seen by both eyes overlaps by approximately 120 degrees. This binocular overlap provides the region within which corresponding information from both eyes can contribute particularly strongly to stereoscopic perception.
The extreme lateral portions of the total visual field are visible to only one eye and are therefore monocular rather than binocular.
Binocular vision should consequently not be imagined as applying equally across the entire human field of view. The human visual field contains both binocular and monocular regions.
Binocular Vision and Depth Perception
Binocular vision provides important information for depth perception, but depth itself is not directly detected by the retina.
A retinal image principally records the directions from which light arrives. A point at one position on the retina could theoretically correspond to an object located at many possible distances along the same visual direction. The visual system must therefore infer depth from additional information.
Binocular disparity and convergence provide two sources of this information, but they form part of a much larger system of depth cues.
Monocular versus Binocular Vision
Monocular vision uses one eye, while binocular vision uses two. The distinction is important, but it should not be interpreted as a division between flat vision and three-dimensional vision.
A person viewing with one eye still perceives an extensive three-dimensional world because numerous monocular cues remain available. These include perspective of form, relative size, foreshortening, occlusion, texture, atmospheric effects, motion parallax, horizon relationships, light and shade and changes of apparent form with viewpoint.
Binocular vision adds further information—especially binocular disparity, stereopsis and vergence—but does not replace these monocular perspective processes.
Binocular Vision Is Most Important at Close Range
The importance of binocular stereopsis varies considerably with distance.
At close range, the difference between the two eye views can provide highly effective information about relative depth. At intermediate distances binocular information can remain useful, but its effectiveness progressively declines as the object becomes more distant.
For extensive landscapes, distant buildings and other large-scale spatial environments, human depth perception consequently depends increasingly upon monocular perspective cues rather than upon stereopsis alone.
This is a fundamental reason why binocular vision should not be treated as the sole or universal explanation of three-dimensional visual perception.
Perspective and Binocular Vision
Perspective and binocular vision are closely related but are not the same thing.
Each eye receives its own perspective view, determined by its viewpoint relative to the spatial scene. The difference between these two views contributes to binocular depth perception. At the same time, each individual eye view contains monocular perspective information such as diminution, aspect, foreshortening, occlusion and convergence.
Human spatial vision therefore combines two perspective images plus numerous monocular and binocular depth cues within a much wider visual and perceptual process.
Binocular Perspective
The term binocular perspective describes visual perspective employing two eyes—or, in an artificial imaging system, two corresponding apertures or viewpoints—to form a stereoscopic view or image.
Natural binocular perspective occurs through ordinary human vision. Artificial binocular perspective can be produced by systems designed to capture, present or reconstruct separate left-eye and right-eye views.
Binocular perspective should therefore be distinguished from ordinary monocular perspective while recognising that both may employ many of the same perspective phenomena and depth cues.
Stereoscopic Perspective
Stereoscopic perspective uses two appropriately related views to generate binocular depth information.
A stereoscopic imaging system may capture the scene through two separate camera positions or apertures corresponding broadly to the two viewpoints of the human eyes. The resulting left and right images are then presented separately to the corresponding eyes.
The visual system can interpret the disparities between the two images in a manner related to natural binocular vision, creating an enhanced impression of dimensional relief and depth.
Binocular Vision and 3-D Images
Binocular stereoscopy is one important way of producing a 3-D image, but it is not the only way a three-dimensional appearance can be experienced or represented.
Paintings, perspective drawings, photographs, conventional films and ordinary television images can all generate powerful impressions of three-dimensional space without presenting different images to the two eyes.
These images depend heavily upon monocular perspective and other depth cues. Stereoscopic systems add selected binocular cues to this existing body of spatial information.
It is therefore misleading to equate 3-D perception exclusively with stereoscopic vision.
Binocular Vision in 3-D Cinema and Displays
Artificial stereoscopic systems reproduce selected principles of binocular vision by providing different perspective images to the two eyes.
Examples include stereoscopic photographs, stereograms, 3-D cinema, some holographic systems, autostereoscopic displays and virtual-reality headsets.
Some systems require glasses or head-mounted equipment to separate the two eye views, while autostereoscopic systems attempt to direct different images to the eyes without conventional viewing glasses.
The realism of such systems depends upon more than stereoscopy alone. Field of view, viewpoint, movement, image scale, resolution, focus, monocular perspective cues and the behaviour of the display system can all affect the resulting experience of depth.
Fixed, Multi-Angular and Moving Binocular Views
Binocular perspective images can differ substantially according to the number and freedom of the viewpoints they provide.
A conventional stereoscopic photograph provides a largely fixed binocular viewing arrangement. Other systems can provide multiple views so that the apparent scene changes as the observer moves. Virtual environments can go further by computationally generating changing left-eye and right-eye views as the observer changes position and orientation.
The resulting experience of three-dimensional space therefore depends upon the complete system of image capture, representation, display, viewpoint and observation.
The Horopter and Binocular Single Vision
The horopter is another concept associated with binocular vision. It describes a curved locus or surface of corresponding spatial points associated with binocular single vision for a particular fixation arrangement.
The horopter illustrates that binocular vision cannot be understood simply as two flat pictures mechanically placed together. It involves changing relationships between the two eyes, fixation, retinal correspondence, spatial position and perceptual combination.
Binocular Vision and Visual Perspective Type 2
Within Perspective Category Theory, binocular vision belongs principally to Visual Perspective Type 2: the human retinal and perceptual subclass of the broader category of Visual Perspective.
Visual Perspective Type 2 includes the processes through which physical and represented spatial reality is experienced by the human visual system. Relevant factors include retinal projection, viewpoint, apparent size and shape, depth cues, binocular perception, visual acuity, eye and head movement and other physiological and perceptual processes.
Binocular vision is therefore one important component of human visual perspective rather than a complete explanation of perspective or spatial vision in itself.
Binocular Vision Is Part of a Larger Visual System
Human vision is an exceptionally complex system in which many optical, physiological and perceptual processes operate simultaneously.
Binocular disparity, fusion and vergence operate alongside retinal projection, eye movement, head movement, focus, acuity, peripheral vision, motion information and numerous perspective phenomena.
Consequently, binocular perception should not be isolated from the larger problem of how the visual system interprets changing perspective images and reconstructs useful knowledge about spatial reality.
Although much is understood about stereopsis and binocular combination, important aspects of binocular perception remain active areas of research.
Binocular Vision — Frequently Asked Questions
What is binocular vision?
Binocular vision is vision using two eyes. Each eye receives a slightly different perspective view of spatial reality, and the visual system combines information from the two views to support a unified visual experience and provide information about depth and distance.
What is binocular depth perception?
Binocular depth perception is depth judgement using information available from the two eyes, particularly binocular disparity, stereopsis and changes in vergence.
What is stereopsis?
Stereopsis is the perception of relative depth from differences between the images received by the two eyes. It is particularly effective for fine depth judgements at relatively close viewing distances.
What is binocular disparity?
Binocular disparity is the difference in the relative positions or forms of corresponding image features seen from the slightly different viewpoints of the two eyes. These differences can provide information about relative depth.
What is binocular convergence?
Binocular convergence or vergence is the changing angular alignment of the two eyes as they fixate objects at different distances. Information associated with this change can contribute to distance estimation.
Can you perceive depth with one eye?
Yes. Human depth perception also relies upon many monocular cues, including perspective, diminution of size, foreshortening, occlusion, texture, motion parallax, aerial perspective, light and shade and horizon relationships.
Is binocular vision the same as 3-D vision?
No. Binocular vision provides important stereoscopic depth information, but three-dimensional perception also depends upon many monocular and perspective cues. A convincing impression of 3-D can therefore occur without stereopsis.
How large is the binocular field of view?
The combined human horizontal field of view is approximately 200 degrees, with a central binocular overlap of approximately 120 degrees. The extreme lateral regions are visible monocularly to only one eye.
Is binocular vision most important for near or distant objects?
Binocular stereopsis is especially effective for close-range relative-depth judgements and becomes progressively less effective as viewing distance increases. Monocular perspective cues become increasingly important for interpreting larger-scale and more distant spatial environments.
Binocular Vision and the Wider Study of Perspective
Binocular vision reveals an important principle of perspective: spatial appearance depends upon viewpoint. The two eyes occupy slightly different viewpoints and therefore receive different perspective images of the same spatial reality.
The visual system uses the relationships between these views, together with monocular perspective, movement, optics and other depth cues, to interpret three-dimensional space.
Binocular vision is therefore an important bridge between perspective, optics, human vision, depth perception, stereoscopy and 3-D imaging—but it is one component within the much larger system through which spatial reality is seen and understood.