Binocular vision refers to the ability to use information from both eyes at once. This allows us to use and compare information from each eye, and more accurately judge distance, coordinate eye movement, and take in information.
A quick look around the animal kingdom will show many different styles of visual systems. Each system is designed to meet the needs of that particular animal.
An interesting thing is to notice the position of the eyes in relation to the head as well as to each other. This gives very good clues about how the visual system is used.
Example: The lighter shades represent areas seen by one eye only, the darker area is seen by both eyes simultaneously.
Notice that the majority of the human visual field is overlapped. This emphasizes how our system is designed to have both eyes working together. This is crucial for our attention to detail and also so that we can navigate through our environment in a smooth and accurate manner (thanks to better depth perception and spacial awareness).
There are a variety of issues in the visual system that can keep this system from working properly:
Some specific ways we use the information
The ability of our brain to accurately construct our perception of our physical environment is an incredibly complex process relying on physiological and psychological cues.
Accommodation is the tension of the muscle that changes the focal length of the lens of the eye.
When watching an object close to us our eyes point slightly inward. This difference in the direction of the eyes is called convergence. This depth cue is effective only on short distances (less than 10 meters).
As our eyes see the world from slightly different locations, the images sensed by the eyes are slightly different. This difference in the sensed images is called binocular parallax. The human visual system is very sensitive to these differences and binocular parallax is the most important depth cue for medium viewing distances. A sense of depth can be achieved using binocular parallax even if all other depth cues are removed.
Monocular Movement Parallax
If we close one of our eyes, we can perceive depth by moving our head. This happens because the human visual system can extract depth information from two similar images sensed one after the other in the same way that it can combine two images from different eyes.
Retinal Image Size
When the real size of the object is known, our brain compares the sensed size of the object to this real size and thus acquires information about the distance of the object.
When looking down a straight level road we see the parallel sides of the road meet in the horizon. This effect is often visible in photos and it is an important depth cue. It is called linear perspective.
The closer we are to an object the more detail we can see of its surface texture. So objects with smooth textures are usually interpreted as being farther away. This is especially true if the surface texture spans the entire distance from near to far.
When objects block each other out of our sight, we know that the object that blocks the other one is closer to us. The object whose outline pattern looks more continuous is felt to lie closer.
The mountains on the horizon always look slightly bluish or hazy. The reason for this are small water and dust particles in the air between the eye and the mountains. The farther the mountains, the hazier they look.
Shades and Shadows
When we know the location of a light source and see objects casting shadows on other objects, we learn that the object shadowing the other is closer to the light source. As most illumination comes downward, we tend to resolve ambiguities using this information. The three-dimensional computer interfaces are a nice example of this. Also, bright objects seem to be closer to the observer than dark ones.
In the majority of cases, the information needed to accurately perceive an environment is available but it just has not been properly assimilated. It is possible to train and hone some of the binocular processing activities in order to improve spacial judgement, sports performance, and skills useful for navigating every day life.