Depth perception
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.
How eyes converge
When your eyes look at a closer object, they angle inwards. This is called convergence. Greater amount of convergence causes eye muscles to do more work. This is a useful feedback indicator to the brain to give us a clue as to how far away an object is.
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.
How we see 3D
Dr Adrian Dorrington, a scientist from the University of Waikato, explains how the brain interprets the images our two eyes receive. The net result is that we experience depth perception. 3D television works in a similar way. By wearing special glasses, two pictures are shown at the same time, one to each eye. The brain interprets this such that we see in 3D.
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.
3D technology
Dr Adrian Dorrington, a scientist from the University of Waikato, explains the basic concept behind 3D television and film. One system requires the viewer to wear active glasses so that the right image is delivered to the right eye at the right time. Active glasses require a power source to operate the shutter system in the lenses.
Time-of-flight 3D cameras
University of Waikato scientist Dr Adrian Dorrington is investigating different applications of 3D cameras. The camera emits light, and the time of flight from camera to object and back again is measured. One application of this type of camera is in assessing the height of customers entering and leaving premises for security reasons.
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.
