Seeing with two eyes helps people to judge distances and to see in 3D, but even using one eye, there are many clues (often referred to as visual cues) to give people depth perception. Depth perception using computers is more difficult.
Binocular vision – seeing 3D with two eyes
There are two main binocular cues that help us to judge distance:
- Disparity – each eye see a slightly different image because they are about 6 cm apart (on average). Your brain puts the two images it receives together into a single three-dimensional image. Animals with greater eye separation, such as hammerhead sharks, can have a much greater depth perception (as long as the view from both eyes overlaps the same scene). This can be very useful when trying to catch fast-moving prey.
- Convergence – when looking at a close-up object, your eyes angle inwards towards each other (you become slightly cross-eyed). The extra effort used by the muscles on the outside of each eye gives a clue to the brain about how far away the object is. If you hold your finger 20 cm in front of your eyes, your muscles need to work a lot harder than when your finger is 50 cm away.
These binocular cues are most effective for objects up to 6 m away. After this, the amount of eye separation does not give a great enough difference in images to be useful.
3D movies make use of disparity by providing each eye with a different image. However, the brain does not receive any cues from convergence as it normally would. This may cause discomfort for some people.
Monocular cues – 3D information from a single eye
If you close one eye, your vision becomes much less three-dimensional, but there are still many clues that allow you to judge distances. You are still able to pick up a pen, move around without crashing into things and even catch a ball.
Some of these monocular cues are as follows:
-
Accommodation
– this is the change of
focus
when you look at a close-up object. The ciliary muscles inside the eye need to work harder to change the shape of the lens inside your eye. The effort required provides the brain with information about distance.
- Sharp
focus
or blurry – if two objects are at the same distance, they will both appear to be in
focus
. Objects that are closer or further away will appear blurry.
- Motion parallax – if you move your head, objects that are close to you will appear to move more than those objects that are further away.
- Superposition – objects that appear to move in front of other objects must be closer (a little obvious perhaps, but very useful). You will often see some animals to move their heads from side to side or up and down. This gives important depth information both for motion parallax and for superposition. Try it out!
- Vividness of colours – distant objects often appear less bright and colourful. This is due to the scattering of light as it travels from that distant object. Having more of the atmosphere to travel through means that light will be scattered more, so the colours will not seem as bright.
- Definition and textures – close objects will have a lot of detail and definition apparent. More distant objects will not appear with as much detail. This is very noticeable when looking at a field of grass. Close up, the blades of grass will be noticeable. Further away, the grass is more of a sea of green.
- Relative size – if we already have an idea of the size of two people or objects in a photo, this can give a good clue as to how far apart they are.
Artists use some of these monocular cues to give a perception of distances in a two-dimensional picture.
Creating 3D for movies, robots and security cameras
Computers and robots do not have brains to process these cues from digital images and interpret 3D information. For them, there needs to be an entirely different technology.
Related content
The article Light – polarisation provides insight on how 3D glasses work.
Activity idea
In the activity, Pinhole cameras and eyes students make a pinhole camera and see images formed on an internal screen. They then use a lens and see brighter and sharper images. This models the human eye.
Learn about the binocular cues for depth perception, and understand the meaning of binocular rivalry and retinal disparity through the binocular cues examples. Updated: 03/09/2022
Shannon has a Ed.D in curriculum and instruction from Oakland City University. She earned her Masters in building level administration from Oakland City University and her Bachelors of Science in biology from Marian University. Shannon transitioned to teaching over 11 years ago. She has experience teaching 6th-12th grade in the areas of general science, biology, and advanced biology.
What are the Binocular Cues for Depth Perception?
When one uses two eyes to see it is called binocular vision. With binocular vision, one can sense the depth of objects. Depth perception, or stereopsis, provides a relationship between the things one sees in their visual field, near or far. Each eye produces an image that is put together in the brain to create a three-dimensional image. These objects appear three-dimensional due to binocular depth cues.
When one looks at the image, they see the depth of the hearts toward the center of the image.
There are two types of binocular depth cues: convergence and retinal disparity. Convergence uses both eyes to focus on the same object. As an object moves close, the eyes come closer together to focus. As the eye look at an object further away, the eyes move further apart to focus. Retinal disparity creates an overlapping image. Each eye produces an image; however, the angle of each eye is different, making the images different from each eye.
What is Binocular Convergence?
Proprioceptive senses rely on the five senses: touch, taste, smell, sight, and hearing. Proprioceptive senses are receptors in the body that help one experience the world around them. In the case of sight, it is binocular convergence.
Binocular convergence is when both eyes rotate inward at different angles to focus on an object. The degree to which the eyes turn is sent to the brain to determine how far away an object may be. Binocular convergence creates a three-dimensional image that helps with depth perception and the location of objects.
Retinal Disparity (Binocular Parallax)
Retinal disparity, or binocular parallax, is one’s sense of depth perception. Depth perception is possible due to each eye seeing at different angles. The eyes are approximately 6.3 centimeters apart, providing two different views of the same object and the environment. Retinal disparity exists in organisms with two eyes that are directed toward the front.
To observe retinal disparity, cover one eye and look at an object, observe the location of the object. Cover the other eye and view the same object, paying attention again to the location of the object. The two different eyes view the same object to exist in different places.
Imagine you’re in a car and you see a tree in the distance. How is it that as we drive closer the tree begins to look bigger? Trees obviously aren’t growing. So what is causing this? I’ll give you a hint… it’s our brain and eyes using depth cues. Depth perception refers to the ability to see the world in 3D and judge how far away/close objects are from and to us. This judgement is very important for navigating everyday life. How we move from one point to another relies quite heavily on our ability to perceive depth, and even picking up an object, such as your pencil, relies on the ability to judge depth.
For example, if we were crossing the road and couldn’t judge how far away a car was, it would be a bit of a disaster.
Let’s take a look at depth cues in psychology!
- We will start by taking a look at monocular depth cues definition psychology and binocular depth cues psychology.
- We will then move on to look at monocular depth cues examples whilst exploring aspects such as height in plane, relative size, occlusion and linear perspective.
- Moving along to do the same and looking at binocular depth cues examples, focusing on convergence and retinal disparity.
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Finally, we will highlight the difference between monocular and binocular depth cues.
Monocular Depth Cues – Definition in Psychology
Monocular depth cues in psychology can be defined as:
Monocular depth cues: information about the depth that can be judged using only one eye. Monocular depth cues can be used in pictures, so a lot of monocular depth cues are used in art to give viewers a sense of depth.
Binocular Depth Cues – Definition in Psychology
Binocular depth cues in psychology can be defined as:
Binocular depth cues: information about depth that uses both eyes to see and understand 3D space; this is a lot easier for our brains to comprehend than monocular depth cues.
The difference between monocular and binocular depth cues is that monocular depth cues use one eye to judge depth, and binocular depth cues use both eyes to perceive depth.
Monocular Depth Cues – Types and Examples
There are four monocular depth cues you will need to know for GCSE psychology. These are:
- Height in plane
- Relative size
- Occlusion
- Linear perspective.
Height in plane
Height in plane is when objects placed higher up appear or would be interpreted as further away. Have a look at the monocular depth cues example below, note that the house that is placed higher would be interpreted as being further away from us, and the house lower down would be seen as being closer to us.
Example of height in plane, Erika Hae, StudySmarter Originals
Relative size
If there are two objects that are the same size (e.g., two trees of the same size), the object that is closer will look larger. Have a look at the monocular depth cues example below, tree number 1 seems closer because it is larger, and tree number 2 seems further away because it is smaller.
Example of relative size, Erika Hae, StudySmarter Originals
Occlusion
This is when one object partially hides another object. The object in front that is overlapping the other is perceived to be closer than the one that is being partially hidden. Look at the monocular depth cues example below; the rectangle appears closer as it overlaps and partially hides the triangle.
