How Does Stereovision Work?
Each eye sees a slightly different image of the world. The brain combines these two images into a single, three-dimensional view - creating depth perception. This process happens automatically and continuously, allowing us to interact smoothly with our environment.
Why Do We Have Two Eyes?
At first, you might think it’s for depth perception, but close one eye and you can still judge distances between objects quite well. Some remarkable people, like the cricketer the Nawab of Pataudi and the aviator Wiley Post (the first person to fly solo around the world), excelled in their fields with just one eye. So why do we have two?
It’s not just about having two eyes; it’s about where they are. Prey animals, like rabbits and pigeons, have eyes on the sides of their heads, giving them a wide field of view to spot danger.
Predators, like tigers and owls, have forward-facing eyes, just as we do.
Forward-facing eyes allow stereoscopic vision, where both eyes work together to detect depth and ‘break through’ camouflage, an essential skill for hunters.
Stereoscopic vision is very good at breaking camouflage
For humans, stereoscopic vision isn’t just about survival. It helps us judge distances, catch a ball or even pour a drink. It’s a powerful ability that we often take for granted, and its absence can have a bigger impact than people realise.
How The Brain Creates Depth
Each eye's image travels along the optic nerve. Because the optic nerves partly cross, information from both eyes is combined in the visual cortex at the back of the brain.
The brain analyses the disparities between the two images (the small differences in where objects appear to each eye) to calculate depth. This process is called stereopsis, from the Greek stereos, "solid", and opsis, "sight". By measuring these disparities, the brain builds a detailed depth map that lets us judge space precisely and interact with the world around us.
Some children are born with conditions such as strabismus (eye misalignment) or amblyopia (lazy eye) that interfere with this process, leading to poor 3D vision or even stereoblindness. Around 1 in 14 adults has no measurable stereovision (Chopin et al., 2019), and studies suggest that as many as 1 in 3 people have reduced stereo vision (Hess et al., 2015). The proportion rises with age: in one study of over-65s, only 27% had normal stereopsis on the Frisby test (Wright & Wormald, 1992).
Two eyes, two views. Looking down at a simple pyramid shape, each eye sees it from a slightly different angle: the inner square sits a little to one side in the right eye's view and to the other side in the left eye's. These small differences, called binocular disparity, are what the brain uses to see the shape in depth.
Where the two views meet. Each eye has its own field of vision (shown in blue and pink). Where they overlap (the dark area), both eyes see the same object from slightly different positions. The brain combines the two images into a single picture with depth: this is stereoscopic, or 3D, vision.
Why is Stereovision Important?
Stereovision plays a key role in everyday activities, including:
- Catching and throwing objects
- Reading and focusing comfortably
- Judging distances when walking, stepping or driving
- Coordinating hand and eye movements
In children, it is especially important for development, learning, and confidence.
Signs a Child May Have Reduced Stereovision (Depth Perception)
Depth perception problems in children are not always obvious. Many children with reduced stereovision can still see clearly on a standard eye chart, but may struggle with activities that require accurate judgement of distance, movement, and spatial awareness.
Some children adapt well and may not realise their vision is different from others. Early testing can help identify binocular vision problems that might otherwise go unnoticed.
Common Signs of Reduced Stereovision
Children with reduced stereovision or depth perception difficulties may:
- Struggle to catch or throw balls accurately
- Frequently bump into objects, doorways, or furniture
- Find stairs or uneven surfaces difficult
- Avoid sports or activities requiring coordination
- Hold books very close when reading
- Tilt or turn their head when focusing
- Close one eye in bright sunlight
- Have reduced confidence in physical activities
- Seem clumsy compared with other children of a similar age
- Experience visual fatigue during close work
These signs do not always mean a child has a stereovision problem, but they can indicate that further binocular vision assessment may be helpful.
Conditions Commonly Associated with Reduced Stereovision
Reduced stereovision can occur when the eyes are not working together effectively. This may be linked to conditions such as:
Strabismus (Squint)
A misalignment of the eyes where one eye may turn inwards, outwards, upwards, or downwards. This can affect how the brain combines information from both eyes.
Amblyopia (Lazy Eye)
Reduced vision in one eye during childhood development, often associated with weaker binocular vision and reduced stereoacuity.
Binocular Vision Disorders
Some children have difficulty coordinating eye movements even when both eyes appear straight.
Following Eye Treatment
Stereovision testing may also be used during or after treatment for binocular vision conditions to assess improvement in depth perception.
Why Early Stereovision Testing Matters
Good stereovision supports many everyday activities including learning, movement, coordination, and confidence. Identifying reduced stereo vision early may help clinicians monitor binocular vision development and support appropriate treatment where needed.
Stereovision testing is commonly carried out by:
- Orthoptists
- Optometrists
- Ophthalmologists
- Paediatric vision specialists
- Some Doctors such as General Practitioners
The Frisby Stereotests are designed to assess stereopsis using real depth targets viewed naturally without special glasses.
How is Stereovision Measured?
Stereovision is measured using tests called stereotests. These tests assess how well the brain combines input from both eyes and how accurately depth can be perceived. Results are typically measured in seconds of arc, with lower values indicating finer depth perception.
Stereovision is routinely assessed by optometrists (opticians), orthoptists, and ophthalmologists as part of binocular vision testing.
How Do Stereotests Work?
Different types of stereotests exist. Some use special glasses to present separate images to each eye. Unlike many stereotests, the Frisby Stereotest uses real physical depth that can be seen naturally, without the need for glasses. This allows testing in natural viewing conditions, makes it suitable for young children, as well as supporting more reliable results.
Stereovision testing can help identify binocular vision problems that may not always be detected through standard visual acuity testing alone.
Kristina Alexander
Senior Orthoptist, John Radcliffe Orthoptics Department, Oxford, UK
When should Stereovision be Tested?
Stereovision may be assessed:
· During routine eye examinations
· If a child shows signs of coordination or visual difficulty
· When a squint (strabismus) is present
· During or after treatment for amblyopia (low vision in one eye)
Early testing can help identify issues that might otherwise go unnoticed.
FAQs
What is Stereovision?
Stereovision — also known as stereopsis — is the ability to see depth and perceive the world in three dimensions. It happens when the brain combines slightly different images from each eye into a single image with depth information.
Good stereovision helps us judge distances accurately and supports many everyday activities, including movement, coordination, reading, and sports.
What is Stereopsis?
Stereopsis is the visual process that allows the brain to combine information from both eyes to create depth perception. It is an important part of binocular vision and helps us understand how far away objects are in relation to each other.
What Causes Poor Depth Perception?
Reduced depth perception can occur when the eyes do not work together effectively. This may be associated with conditions such as strabismus (squint), where the eyes do not align properly, amblyopia (lazy eye), or other binocular vision problems.
Some children with reduced stereovision may still pass a standard eye chart test used to measure how clearly each eye can see. This is because standard eyesight tests mainly assess visual acuity, rather than depth perception and binocular vision.
How is Stereovision Tested?
Stereovision is assessed using specialised stereotests that measure how accurately the brain combines visual information from both eyes.
Different stereotests use different methods. Some use special glasses, while others — such as the Frisby Stereotests — use real depth targets viewed naturally without glasses.
At What Age Should Stereovision be Tested?
Stereovision is often assessed during childhood eye examinations, particularly if there are concerns about coordination, eye alignment, reading comfort, or visual development.
Early assessment can help identify binocular vision problems that might otherwise go unnoticed.
Can a Child Have Poor Depth Perception with Normal Eyesight?
Yes. A child may be able to read a standard eye chart clearly but still have reduced stereovision or binocular vision difficulties.
Because stereovision depends on how both eyes work together, standard eyesight tests do not always assess depth perception fully.
Who Typically Tests a Child's Stereovision (depth perception)?
Stereotests are commonly used by orthoptists, optometrists, ophthalmologists, and paediatric vision specialists.
In some areas, children may receive vision screening during their early school years or at nursery. If there are concerns about a child’s vision, depth perception, or eye coordination, an appointment with an optometrist (optician) or general practitioner (GP) is often a good first step.
Do All Stereotests Require Glasses?
No. Some stereotests use special glasses to separate two different images presented to each eye. Others, including the Frisby Stereotests, use real depth targets that can be viewed naturally without glasses.
This allows testing in more natural viewing conditions and can be particularly useful for young children.
More About Frisby Stereotests
Explore stereotests designed for assessing stereopsis and depth perception in children and adults.