Stereopsis (depth perception) is the visual ability to perceive the world in three dimensions (3D) - length, width, and depth - which then allows a person to judge where an object is relative to him or her.
Depth perception arises from a variety of visual stimuli referred to as depth cues. These cues may be monocular (single-eye) or binocular (two-eye) cues to depth. You could also use the word "clues" for cues as these are the "clues" that tell the visual system about the 3D components of an object or space.
Monocular cues include:
Binocular cues include:
What's very interesting about vision is that crude depth perception does not require the use of both eyes. Take a moment to perform a quick experiment. Cover one eye with your hand and look around. Are you able to tell what items in the room are closer to you? Farther away? The answer should be yes! Even someone with one eye is able to use single-eye cues to depth.
This is probably the easiest one to understand. You learned when looking around in the world that some objects are big and some are small. When a big object (say a car) appears small, you have a good idea that this isn't really a small car, but rather a car that is far away. This the idea of relative size.
Overlap (also called interposition) is a cue that tells the visual system that the front-most object is likely closer because it blocks the view of the back-most object.
This is a fun drawing trick usually learned at an early age. Think of two lines that are parallel (lines on a road or railroad tracks are a good example). As you look into the distance, the lines appear to get closer and closer, which gets interpreted as the lines being farther away.
On a broader scale, we learn from young age that the earth is brown or green and the sky is blue. The "blueness" of the sky is due to scattering of shorter wavelength light (Rayleigh scattering). The blue outline of an object against the sky gives a cue that the object is farther away.
These are grouped together as light or shadowing of an object becomes a depth cue. Our visual system assumes light comes from the top-down of an object - changing how light and shadows appear on an object can change how the depth is interpreted.
Perform another simple experiment. Close one eye. Now hold your finger or thumb up at arms length away. Look past your finger or thumb at a distant object and shake your head gently from side-to-side as if saying "no". What happened? Did you notice your finger moving slightly in the opposite direction of your head movement? Now keep your finger out at arms length and use your other hand to do the same motion, but at a distance closer to your nose. Focus on your far away finger and rock your head left and right. Do you see the nearer finger moving faster than before? This is the parallax effect. Objects that are closer move across the retina at a faster velocity than distant objects when moving.
Binocular depth cues are more complicated and required a coordinated effort from both eyes. The eyes must both move in the correct direction so that the visual axis of each eye is pointing at the same object of interest, and the images must be able to be fused.
Fusion is the ability of the visual system to take two similar objects and combine them into a single object. Flat fusion has no cues to depth - these are simple objects that look flat on a surface. The next level is stereopsis.
Stereopsis is the highest (most difficult) level of extracting depth information from the visual world. The eyes must have a relatively similar image (this is why patients with lazy eye or strabismus have difficulty with stereopsis - the brain is working with dissimilar quality of images!). Due to the front-facing location of our two eyes, each eye sees a very similar, yet slightly offset image when looking at an object in space. This slight offset is termed retinal disparity. The brain can then interpret this offset as a binocular depth cue.
Stereopsis can be broadly classified into two types - coarse stereopsis and fine stereopsis. Coase stereopsis is large, more easily distinguishable amounts of depth using retinal disparity cues. Fine stereopsis is often what is tested in an eye exam - this is very fine amounts of depth between objects. What is interesting is that the human visual system appears to have developed two different neural mechanisms to extract these different forms of depth information from the visual world.
Random Dot vs. Contour Tests There are different methods of stereo testing targets. Random dot targets at first glance look like a bunch of scattered dots. These targets require the patient to combine the images to see a shape or pattern. Countour targets have a distinct shape but use smaller and smaller offsets of the shape to measure stereo ability. Most tests require some form of dissociation, either with polarized lenses or with red/green or red/blue glasses.
The Titmus stereo test consists of a combination of contour targets. The most common targets are a series of rings for older patients, animals for children, and a large stereo fly that is used mainly for screening. The circles test from 800 down to 40 seconds of arc while the animals test 400 to 100 seconds of arc. The fly has variable stereo from the head to the thorax and contains disparity values from about 700 to 400 seconds of arc.
The TNO stereo test uses a random dot pattern. Red and green glasses are required to dissociate the patient. The test contains multiple plates with hidden objects and can measure disparity values down to 15 seconds of arc.
The Lang stereo test is unique in that no dissociating glasses are needed. This test is a random dot test that can test disparity values of 1200 arc seconds to 200 arc seconds.
Similar to the Lang test, the Frisby test does not require the patient to wear dissociating glasses. Instead, the test uses transparent plates with different thicknesses to alter the disparity in the test. The test can be performed at different distances, which changes the disparity of the test. The test can measure values between 600 and 20 arc seconds.
A number of stereo tests include the use of random dot stereo targets that are viewed with polaroid glasses. Some tests, such as the Random Dot E, use single plates for testing, while others may use shapes or animals embedded in the test. The level of disparity tested varies from test to test.
Michael Kalloniatis and Charles Luu. The Perception of Depth. https://www.ncbi.nlm.nih.gov/books/NBK11512/
Deborah Giaschi. Sathyasri Narasimhan, Aliya Solski, Emily Harrison, and Laurie M.Wilcox. On the typical development of stereopsis: Fine and coarse processing. https://en.wikipedia.org/wiki/Depth_perception
https://www.sciencedirect.com/science/article/pii/S0042698913001867 Chopin A, Bavelier D & Levi DM. The prevalence and diagnosis of ‘stereoblindness’ in adults less than 60 years of age: a bestevidence synthesis. https://www.ncbi.nlm.nih.gov/pubmed/30776852