Depth Perception and Stereopsis: What It Is, How It's Tested

Updated 2026-10-23

Written by the Optics Trainer team from published research, with every source linked below. Not yet reviewed by a clinician.

Depth perception is your ability to see in three dimensions and judge how far away things are. You use it to pour a drink, catch a ball, park a car and step off a curb. Part of it comes from having two eyes. Each eye sees from a slightly different angle, and the brain turns that difference into a fine sense of depth called stereopsis. You also get depth clues from one eye alone. This guide explains what stereopsis means, how a depth perception test works at the eye doctor, what the results in seconds of arc mean, what causes depth perception issues, and what studies show about improving stereopsis.

What is stereopsis? Depth perception explained

The American Academy of Ophthalmology (AAO) describes depth perception as seeing in three dimensions and judging distance. Accurate depth perception usually needs both eyes. When both eyes see clearly and the brain blends their two views well, the result is stereopsis.

Stereopsis works because your eyes sit a few centimeters apart. An object nearer or farther than the point you are looking at lands on slightly different spots in each eye. The brain reads that small mismatch, called binocular disparity, as depth. EyeWiki, the AAO's clinical wiki, calls stereopsis the highest level of binocular vision. It also notes that "Stereopsis is not present at birth and develops in a child at 3-6 months of age." Problems with vision in the first 8 years of life can affect how it develops.

Depth perception with one eye: monocular cues

You can still judge depth with one eye, using clues that do not need two eyes. EyeWiki lists several of these monocular cues:

Relative size: a familiar object looks smaller when it is farther away.
Overlap: a nearer object blocks part of one behind it.
Perspective and texture: parallel lines seem to meet in the distance, and patterns look finer farther away.
Haze, color and blur: distant things look hazier and less sharp.
Movement: when you move your head, near objects shift more than far ones.

Depth perception issues: causes and everyday effects

Monocular cues help, but EyeWiki notes they are easier to fool with optical illusions than two-eyed depth. The AAO says some people who have relied on one eye for a long time have acceptable depth perception because their brain has adapted.

The AAO lists conditions that can cause depth perception problems: blurry vision in one eye, an eye turn (strabismus), lazy eye (amblyopia), a nerve problem in one eye, and an injury to one eye. EyeWiki adds a cataract present from birth that is not treated early, and a large difference in glasses prescription between the eyes.

A 2018 review in Clinical and Experimental Optometry looked at how reduced stereopsis affects daily life. People with poor or absent stereo were slower and less accurate when reaching for and grasping objects, and walked more cautiously around steps. Abnormal stereo has been reported as a risk factor for falls in older adults. The authors also said there is little research on how missing stereopsis affects quality of life, work and hobbies.

Stereopsis tests at the eye doctor

A depth perception test at the eye doctor is quick. You usually wear special glasses and look at a booklet or card held at reading distance. Each eye sees a slightly different version of the picture, and if your brain blends them, part of the image seems to float above the page. Common stereopsis tests include:

Common clinical stereopsis tests
Test	Glasses	What you see
Titmus (stereo fly)	Polarized	A large housefly whose wings seem to stand up, plus finer circles and animals
Randot and Randot Preschool	Polarized	Shapes hidden in a random-dot pattern that appear only with both eyes
TNO	Colored filter	Random-dot plates with hidden shapes
Lang	None	Pictures hidden in a dot card; useful for young children because no glasses are needed

What stereopsis test results mean, and what online tests can't check

Stereopsis results are given in seconds of arc, a very small unit of angle (one second of arc is 1/3,600 of a degree). The number is the smallest depth difference you could see. A smaller number means finer stereopsis. EyeWiki puts the Titmus fly at about 3,000 seconds of arc and the finest Titmus circle at about 40. Eye care professionals can compare the tests in our clinician article on stereoacuity tests.

There is no single normal score that applies to every test. The 2018 review says a result is specific to the test used and the person's age. Even a newer edition of the TNO test gives worse scores than the original. One example of published norms: in a 2008 study of more than 4,300 healthy children using the Randot Preschool test, average stereoacuity improved from 100 seconds of arc at age 3 to 40 at age 7. The study's lower limit of normal was 400 at age 3, 200 at age 4 and 60 at age 7.

Results can also vary from visit to visit, especially in young children. And EyeWiki notes that the Titmus fly may give monocular clues, so a person with weak stereo can sometimes still find it. This is why an examiner may use more than one test and look at the results alongside the rest of the exam.

Online depth perception tests and 3D picture tests can be fun, but they cannot replace an exam. The American Optometric Association (AOA) says online and kiosk vision tests are not a substitute for a comprehensive exam and may not check eye health; a comprehensive exam also looks at how your eyes focus, move and work together. Screens also have limits: the 2018 review works out that on a typical 1920x1080 desktop monitor viewed from 3 meters, a one-pixel shift equals about 18 seconds of arc, which limits how fine a difference a simple screen test can show. If you are worried about your depth perception, the AAO advises seeing an ophthalmologist. For eye teaming problems, see our guide to binocular vision and eye teaming. For lazy eye and eye turns, see our guides to lazy eye exercises and strabismus exercises.

Stereopsis treatment: can depth perception improve?

Stereopsis is usually not treated on its own. It tends to improve, or not, when the underlying problem is treated. What the studies report:

Lazy eye: in a study of 85 children treated with glasses and then patching, 45% improved their stereo by a meaningful step. Before treatment, 66% had no measurable stereo; afterwards, 43% still had none.
Lazy eye from unequal prescriptions: a pooled analysis of 633 children in PEDIG studies found that better stereo after treatment went with better stereo before treatment and better final vision. Even when vision recovered, many children still had poorer stereo than other children their age.
Binocular (dichoptic) treatment: a 2025 analysis of 185 children found stereo outcomes similar to those reported for patching, with no clear extra benefit for two-eyed function.
Eye turns: the 2018 review says that in infantile esotropia, a turn that starts in the first months of life, earlier treatment gives better stereo. Eye turns that come and go carry a lower risk of losing stereopsis.
Adults: EyeWiki mentions a neurobiologist who gained stereovision as an adult, which is one person's experience.

Where Optics Trainer fits

Optics Trainer is a set of game-based vision and brain training apps for iPhone, iPad, Android, web browsers and Meta Quest. It includes depth games, eye teaming games and binocular games where each eye gets its own target while both stay open, using red-blue glasses on a phone, tablet or computer, or built-in separation on Meta Quest. It does not measure stereopsis or test depth perception, it is not a treatment or diagnosis, it is not FDA cleared, and it was not tested in the studies described above.

Optics Trainer is not a substitute for professional diagnosis or prescribed care. Consult your eye care professional for guidance about your specific condition. If your clinician agrees that home practice fits alongside your plan, mobile and web access is $14.99 a month or $149.99 a year. The phone and tablet apps start with a 7-day free trial, and a free web account includes 3 plays a day. VR access is $19.99 a month or $199 a year after a 7-day free trial.

Frequently asked questions

What is the definition of stereopsis?

Stereopsis is the fine depth perception that comes from using both eyes together. Each eye sees an object from a slightly different angle, and the brain turns that small difference into a sense of depth.

How do eye doctors test depth perception?

You usually wear polarized or colored glasses, or none for the Lang test, and look at a booklet at reading distance. If both eyes work together, parts of the picture seem to stand out from the page. Common tests are the Titmus fly, Randot, TNO and Lang.

What is a normal stereopsis score?

It depends on the test and your age, so there is no single number. Results are in seconds of arc, and lower is better. In a large study of the Randot Preschool test, the average was 100 seconds of arc at age 3 and 40 at age 7. Ask your eye doctor how your result compares for the test they used.

Can you have depth perception with one eye?

Yes, some. One eye can use clues such as relative size, overlap, perspective, haze and motion. These clues are easier to fool than two-eyed depth, but the AAO notes that some people who have relied on one eye for a long time have acceptable depth perception.

Can depth perception be improved?

Sometimes, mostly by treating the cause, such as lazy eye or an eye turn. In one study of children treated for lazy eye, 45% improved their stereo; in another, many treated children still had poorer stereo than their peers. For adults, EyeWiki describes one person who gained stereovision as an adult.

Are online depth perception tests accurate?

They cannot replace an exam. The AOA says a quick online or kiosk vision test is not a substitute for a comprehensive exam and may not check eye health, and a home screen is not set up and checked the way a clinical stereo test is. See an eye care professional if you are concerned.