What is stereoscopic vision?

Stereoscopic vision refers to the ability of humans to see the same scene with both eyes in slightly different ways. It results in our ability to visually perceive depth and distances. Stereoscopic vision is not synonymous with depth perception, but leads to it.

In humans and animals with stereoscopic vision, each eye captures a slightly different image. This difference is known as binocular disparity or retinal disparity. The brain processes these two images in a way that allows us to see slightly around solid objects without moving our heads. It does this by essentially combining the similarities in the two images and then factoring in the differences in our perception of a scene. These differences are usually small, but can translate to a significantly different end result.

The visual advantages that humans have as a result of stereoscopic vision are more obvious compared to someone who does not have this ability, because they have lost the use of one eye, for example. These people can make certain adjustments to account for the loss of depth perception, but it is largely impossible to recover all that has been lost, regardless of these adaptations. Stereoscopic vision is also related to our ability to manipulate small objects with our hands. Similarly, some forest animals use their stereoscopic vision to accurately navigate through branches and other forest environments where accurate depth perception is a matter of survival.

In fact, it is speculated that our stereoscopic vision also evolved as a means of survival, allowing us to see and assess potential threats with greater accuracy and faster response time. In our own time, this aspect of our vision facilitates many routine daily activities. For example, a surgeon must have stereoscopic vision to perform a procedure accurately, and a car driver must be able to determine how far his car is from other objects. Even a task as routine as climbing a flight of stairs would be significantly affected without stereoscopic vision.

As useful as stereoscopic vision is, it is not the only way to judge distance. Our brains can also use what's known as an object's focal length to estimate how far away it is. In doing this, the brain judges distance based on how the eye's lens must change to clearly focus on a given object. This gives a general idea, but is not as precise as stereo vision.

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