Depth perception is the ability of the human eye to see in three dimensions. It is often demonstrated with the measurements of length, width and height. For example, it is easy to recognize that an image is a photo, since cameras do not have accurate depth perception. That’s why when we take a picture, we know the person depicted in the image doesn't have a vase growing out of her head, but the resulting photo makes it appear that way. Optical illusions such as this illustrate how our minds perceive the visual imagery our eyes send to our brains.
Humans have a type of vision that is described as stereoscopic. The term stereoscopic comes from the Greek word stereos, meaning solid. Each eye sees an image that is separate from the image seen by the other eye. The brain processes the images and combines them into the final image seen by the brain. Eye depth perception is determined by how well both eyes function separately and how they function together to interpret images. It can be altered by changes to either eye.
One way we measure how a person sees the world is via a depth perception test. This test measures how you interpret binocular cues. These tests also measure monocular depth perception, or how you see the world when looking with only one eye. Of these cues, what allows humans to see and estimate distances are the ability of both eyes to focus on one object, the ability of the eyes to simply focus, and the ability to recognize shapes and sizes. These tell the brain where objects are in relation to other objects.
Not everyone sees optimally. People suffering from amblyopia, optic nerve hypoplasia and strabismus often have reduced depth perception. A person with an injury to one eye, or a person missing one eye, may not be able to tell where objects are in relation to others. Visual therapy may help improve these problems.
Depth perception plays an important part in many activities. Driving, sewing, threading a needle, watching 3D movies and even walking on uneven ground all require certain levels of depth acuity. People without functioning stereoscopic vision may not be able to perform these activities or may struggle with them. If you suspect a vision problem, speak to a licensed ophthalmologist or optometrist about testing your vision and the options available to correct or accommodate your vision needs.