How Depth Perception Works

Depth perception is the ability to perceive the world in three dimensions (3D) and to judge the distance of objects. Depth perception is achieved when the brain processes different pictures from each eye and combines them to form a single 3-D image. Depth perception makes it possible for the eyes to determine distances between objects and to tell if something is near to us or far away.

In order to have depth perception, you must have binocular vision, also known as stereopsis. Having two eyes allows us to have binocular vision. If someone lacks stereopsis, they are forced to rely on other visual cues to gauge depth, and their depth perception will be less accurate. Although we use other cues in our environment to have depth perception, the most important one is by having binocular vision.

Monocular cues allow us to have some sense of depth perception when true binocular stereopsis is not possible. 

• Motion Parallax: Motion parallax occurs when we move our head back and forth. Objects at different distances will move at slightly different speeds. In physics class, we learn that closer objects move in the opposite direction of our head movement and objects farther away move with our heads. Our brains perceive this and it gives us cues on depth perception.
• Interposition: When objects overlap each other, it gives us monocular cues on which one is closer or farther away.
• Aerial Perspective: Color and contrast cues tell give us clues to how far away an object might be. For example, when light travels from a distance, it is scattered. Scattered light blurs the outlines of things we see and our brain interprets this as being farther away.

A lack of depth perception can affect some key areas that are very important in life. A lack of depth perception can be caused by conditions such as amblyopia, optic nerve hypoplasia, and strabismus. People with only one eye do not have depth perception, as depth perception requires two working eyes.

Having a comprehensive eye examination is the first step to fully assessing your depth perception. An optometrist or ophthalmologist will first asses your vision by measuring your visual acuity or the quality of your vision. If one eye is very blurry and one eye is not, your depth perception will be limited.

When an optometrist checks your eye muscles, he or she will perform a cover test. A cover test measures how well your eyes work together and will check for the presence of strabismus. Strabismus, such as esotropia or exotropia, is an eye muscle problem where the eyes do not work well together, or when one eye is turned in, out, up or down. If the eyes are not in perfect alignment with each other, double vision occurs, or more commonly, suppression. In order for you not to experience double vision if your eyes are not lined up, your brain will suppress the image out of the turned eye. As a result, you are really only using one eye. When this happens, you will not have good depth perception.

Fine levels of depth perception are measured using tests, such as random dot stereograms or contour stereotests. Some of these tests are designed for young children with cartoon characters so that the doctor can easily tell if the child is using binocular vision.