Elementary Binocular Physiology
| This section has been simplified and repetition is used to get the concepts across.
Animals with two eyes facing forwards see the world in 3 dimensions.
Animals with eyes facing sideways don't see good 3D, sometimes none, but they do see a great panorama
Panoramic versus Stereoscopic
Both eyes seeing the same scene reduces the field of view and allows other animals to sneak up undetected. Prey animals prefer a panorama view of the world, where their eyes are placed on each side of the head, allowing nearly a 360 degree field of view (rabbits).
Humans are modified tree apes (except in certain USA states, which deny Darwinism). Tree apes swing from branch to branch and failure to instantly judge the world in 3D could lead to a nasty fall.
Just as with a camera, light rays cross over in the lens so the picture on the retina is upside down and back to front. The brain has no problem with that and just interprets the information around the right way.
Because our eyes are separated by about 65mm, the world seen by each retina is slightly different. The difference is great for objects nearby, but imperceptible for distant things, like mountains. Useful stereoscopic vision while we are moving about fades out about 200 meters away, (1 arc minute) (In the laboratory, stereo vision can be measured at best down to 5 arc seconds: meaning stereo vision theoretically extends to 2.7Km). In standard stereoscopic photography, useful 3d stops short of 200 meters, but can be extended far beyond by using hyper-stereoscopic photography.
A 3D perception of the world is built up in the brain, which computes the retinal differences and interprets them as 3D. People looking at a stereoscopic picture may find the third dimension only develops slowly. The longer they look, the more computation the brain gets through and the 3D impression gets better with time. Those who look at many 3D pictures seem to handle the computations increasingly rapidly, so stereoscopic vision is partly a learned response. In fact, all visual interpretation is learned in infancy, especially binocular interpretation. The basic brain mechanisms are present from birth in the visual cortex, but only experience leads to binocular interpretation becoming "hardwired". If one eye has poor muscles ("cross eyes") it is suppressed and binocular cues may never be worked out properly, which means a squint must be treated early in childhood.
If you hold a pencil in front of your nose and look into the distance, you see two pencils. Some people do not see two pencils because their brains suppress information from one eye. The suppressed eye may shift from right to left, or there may be permanent suppression of one eye. These people may have difficulty seeing 3D pictures. People with only one eye, or with one eye suppressed, never will see in 3D.
Components of stereoscopic vision:
Slight retinal differences, still remaining after focus and convergence, are detected by retinal receptors which are matched in the two eyes.
3D photography gives an impression of depth which is less convincing than real life.
The ability to move our heads sideways to sort out problem areas, by movement parallax, is impossible when viewing photographic stereo pairs. Better stereo images are kept relatively simple, to allow for the reduced information in still photography compared with reality.
The focus of the eyes is adjusted by changing the shape of the lens through the ciliary muscles. As the eyes converge to look nearby, the eyes automatically focus close to prevent the world becoming a blur. The eyes work on "auto focus", just like a modern camera.
But focus does not give 3D information. If the convergence information about distance does not agree with the sharpness of the image, the lens changes shape independently, until the blur is removed. Focus is involuntary, and some people have difficulty with it. Stereo pairs are always presented on a flat surface and so focus information and binocular information are never in agreement, which is a defect of stereoscopic displays.
We interpret the scenery as one image, which is referred to as the "cyclopean eye view of the world". This conceptual cyclopean eye sits in the middle, between our two real eyes.
Neuro-physiologists have found retinal differences between the two eyes are directly mapped in the brain. The occipital visual centres detecting retinal disparity between the eyes are closely linked to neural centres which control eye convergence and focus, by directing the eye muscles.
The brain is tolerant of small differences between the two eyes. Even small magnification differences and small angles of tilt are handled, without conscious double vision. Differences in brightness are handled easily. Differences in contrast are interpreted as changes in reflection angle, producing a sheen on surfaces. Differences in colour are no problem, in fact one eye can see red and the other blue/green and still interpret the scene as three dimensional (as in anaglyphs).
The eye has a tiny area of maximal acuity (the fovea or macula on the retina). The fovea is devoted to high resolution and colour reception.
The background image on this page shows the macula of the author's eye. It is the dark, central area in the retinal photograph. Note how blood vessels come close to the macula but do not cross it, producing a radial pattern. This means the steadily narrowing blood vessels do not interfere with central vision. (False colour image to enhance blood vessels. Original retinal photograph by Bryre Murphy.)
Good stereo discernment depends on good vision because the differences between each eye image can be subtle. Keep your correcting glasses on when viewing in 3d.
3D is seen with reduced accuracy away from the fovea, in the more peripheral vision. Here there are colour-blind receptors (rods) wired up to specialise in detecting differences over time rather than spatial resolution. Movement is detected by changes in our peripheral vision, which is essential to avoid predators (and motor cars). All we know is something in the corner of our visual field changed. Then our eyes turn to find out what changed, by putting it on the fovea, to judge it in high resolution 3D.
Stereo vision does not depend on recognition of objects by each eye. Random dot stereograms reveal 3D is seen first and recognition of objects comes second. However, the world is not all random dots and in the real world we do recognise objects first and see 3D later, even if the more slowly developing 3D impression does not depend on object recognition.
The eyes are connected to 6 other nerve centres in the brain. For example, the balance system. The position of the head is monitored by semicircular canals in the ears. The head and eyes are turned to correct for changing head position, as measured by the ears. If information from the eyes does not match positional data from the ears, some people become nauseated (motion sickness) and their eyes flick from side to side (nystagmus), trying to make the conflicting information coherent.
People with one eye can still get a 3D impression of the world by moving their heads sideways and noticing how objects move in relation to each other. Close objects move, distant objects seem to stay still.
People with two eyes also use this trick, which is well illustrated by looking out the side window of a car or train.
One of the deficiencies of stereoscopic photography is the inability to use head movements to resolve problems of 3D perception. A good example is yacht masts in a marina. Often a mast will vanish behind a closer mast in one eye but not the other. That is resolved by slightly moving the head sideways, but on a stereoscopic photograph the "mast illusion" cannot be solved.
Cross your eyes or go wall eyed to fuse this image. You will see a combination of stereoscopic and double vision, which work together as your eyes move about to build up a 3D impression.
Birds need both:
Except for owls and raptors, bird's eyes are on opposite sides of the head, for a wide angle view.
Looking at a bird from the front, it is usually possible to see parts of both eyes. In this limited angle, which is common to both eyes, can birds see in 3D?
Some birds can, but ordinary chooks cannot see straight ahead. Fowls cock their heads to one side and bob about. This provides movement depth perception for locating that tasty morsel they wish to peck.
The 3D vision part of the human retina is split vertically and on different sides for each eye. The nasal half view of one eye combines with the temporal half view of the other.
One finger test
Hold up one finger about 30cm away and then look at a distant tree. If you gaze at the tree you will seem to have two fingers. Look at the finger and there will seem to be two trees.
This is double vision or diplopia.
Now put a spot on your house window and look through the window, past the spot, at a distant tree. Move back until you only see one window spot when looking at the tree. An ordinary room is not long enough to prevent you seeing two spots.
This means in every-day life we normally see double, but are not aware of it.
You only see one window spot when you move 18 meters back from the window. Once fusion occurs, the angle between the eyes, measured at the spot, is 12 arc minutes. Or 12/60 = 0.2 of a degree.
Beyond 18 meters, the eyes are operating in Panum's fusional zone.
In Panum's zone we can still see there is a difference. The spot seems in front of the trees. This is stereo vision. Stereo vision does not cut out completely until the spot is 2.6km away. (0.000138 degrees, 5 arc seconds.) Stereo acuity varies with measurement techniques and is often lower for parallel stereo (U stereo) at 14 arc seconds. ( 900 meters). These laboratory results are not matched in ordinary vision where stereo seems to become rather useless at 200 meters away. (1 minute of arc). Stereo photographs do not give useful 3D beyond about 200 meters, unless hyper-stereoscopy is used.
The small Panum zone, of 3D without double vision, moves about as we flick our eyes between objects. We get great stereo just in front and just behind an object we are currently fixating on.
If you manage to gaze fixedly at an object, gradually 3D impression and even vision itself fades. Vision depends on our eyes flicking about (saccadic eye movements). Our field of sharp vision is so small (at the macula) saccadic eye movements are also necessary to have a clear idea of our surroundings, but it is a brain-computed clear idea and not simultaneous all over the field of view.
Do not be surprised if you see double when looking at a stereo pair. In fact, if you do not see double there is something wrong, but photographers often help you by making scenery grade slowly into the distance. This gradual shift does not seem to cause so much double vision, although that is an illusion. You mainly see double if an object nearby sits beside an object far away so they are not in the same Panum zone.
Some stereo books say double vision stops at 2 meters and stereo photographs should only be taken of objects lying 2 meters to infinity from the camera, to avoid diplopia. Certainly the diplopia is not so obvious after 2 meters, but the window spot test shows those who think double vision has stopped beyond 2 meters are deluded.
Three finger test
Hold three fingers in front of your nose. This blocks out the nasal visual field of both eyes. (Because light rays cross over in the lens of the eye, the nasal visual field is actually seen by the temporal half of the retina)
You still see a wide-angle view of the world. Move your fingers nearer or further until there is no gap in the world. The brain has fused the two nasal halves of your visual field into one panoramic view.
Diplopia and stereo vision work together,
Two hand test
Place your flat, opened left hand vertically on your nose and angle it left until your left eye only sees the left half of your visual field. (The temporal half.) (Remember, light rays cross over as they pass through the lens and it is actually the nasal half of the retina which is being stimulated).
Now place your flat right hand on the right side of your head and angle it until your right eye only sees the left half of the visual field. (The nasal half)
Open both eyes. You cannot see to the right, but as you look to the left, your vision is perfect 3D.
These two tests show 3D vision needs the nasal half of one eye and the temporal half of the other eye combined to make 3D.
If you only have the two nasal halves of the visual field (3 finger test), panorama vision is maintained but stereoscopic vision is lost.
Although the world detected by our visual system is "a game of two halves," the brain fuses them together to make a single wide view of the world.
Binocular vision is not only useful for panoramic and stereoscopic views of the world. Microscopists have long known that visual acuity, contrast and brightness is enhanced if two eyes are used rather than one. Astronomers find the same thing when viewing paired astronomical photographs in a stereoscope. An amazing binocular telescope, made in New Zealand by Dave Moorhouse, gives enhanced views of the heavens in real time. Stellar astronomy has no real stereoscopy (the stereo base is several AU too short) but stars and nebulae look better when binocular information is integrated in the brain. (3D nebulae on this web site are simulated 3D, but the stereo views of the sun are true 3D.)
ChromaDepth® glasses have micro-optical prisms which deviate colours differently to produce stereo parallax based on colour. Since rainbows are caused by the same process, it is no surprise that the stereo depth follows the colours of the rainbow (ROYGBIV). Red comes forward and blue moves back. Underwater photography is the example quoted for a natural ChromaDepth image, since objects further away become bluer.
The eyes are continually scanning:
flicking across the scenery. (Saccadic eye movements).
The monochromatic nature of 3D vision explains how red/green or red/blue glasses allow a 3D impression when we look at anaglyphs, or go to those stereo movies which depend on the audience wearing coloured glasses.
People who are red/green colour blind can usually see red/green anaglyphs. The function of the coloured filters is to separate the monochrome information into two channels and the actual colour does not matter.
(We will see later how anaglyphs can seem to have true colours, when viewed without coloured glasses.)
Stereoscopic vision is complex:
Check your 3D vision here
There are four main problems.
Often the double vision which results from poor eye muscles is intolerable and the only way the brain can handle it is to suppress one eye. If that happens in infancy, requirement 4 is not met.
When suppression happens in infants, it may become permanent and they may never learn to see in 3D.
Recently it has been discovered things are not quite so hopeless as once thought and after good surgery, good glasses and a teaching program, binocular vision can be restored in children older than 5. See here
Binocular vision problems in children can often be restored with prism lenses. Symptoms caused by impaired binocular vision include mixing up the letters d and b, transposing letters, slow reading, poor writing and headaches. Prism lens correction even as low as 2cm/meter (which was previously regarded as too small to worry about) is said to help these symptoms in about 2/3 of affected children. Even attention deficit disorders (ADD), especially in children who hold their heads at an angle while attempting to correct distorted vision, are alleged to be helped by prism lenses.
Eyes at rest do not always line up the same. You can check this by looking at an object while holding one hand in front of one eye. Rapidly flick the hand sideways to block the other eye, uncovering the first. You may see objects jumping, tilting, or even going out of focus. If you wear glasses, this maneuver may reveal errors in the prescription, especially if the magnification or focus changes. When both eyes work together, such differences between the two are corrected by the eye muscles.
Viewing pictures in 3D is not harmful to normal people (but special conditions apply for children with eye problems). Some people may get headaches at first, but the eye exercises involved in normal stereoscopic perception are mild and may even be beneficial for those with weak eye muscles. Any tendency to headaches disappears with experience. Sleep cures the symptoms, so beginners should practice 3D vision before going to bed. Literature on binocular therapy
Headaches and even nausea are worse with cross eye stereo (X stereo).
Well aligned parallel eye stereo, using lenses which place the focus at infinity (weak magnifying glasses) relaxes the eye muscles. Looking at U stereo can be a good exercise for eyes tired at work by prolonged convergence onto a computer screen. Nearly, but not as good, as taking the dog for a walk in twilight, which also relaxes the eyes and neck muscles. Unfortunately, anaglyph viewing is not as relaxing as U stereo, since the eyes are not brought to full parallel and the coloured glasses are not that comfortable. Anaglyphs are a way to show that 3D is possible, they are not the ideal viewing method.
Children who are cross eyed SHOULD NOT ATTEMPT CROSS EYE STEREO. They are prescribed eye exercises designed to diverge the eyes. They should only use parallel eye stereo on this web site. Get advice from your ophthalmologist / optometrist first. A good mirror viewer is recommended (such as the Screen Scope) to help such children. Note that full parallel stereo with the images on this web site and a Screen Scope needs an 800x600 pixel screen, which you can modify as necessary in "screen properties" or Mac Preferences.
Persistence of vision
An excellent method for 3D viewing on a computer screen depends on glasses which rapidly change from clear to black. This can be arranged so the computer shows the left eye picture while the left eye glass is clear but the right is black and vice versa. If this happens fast enough (e.g. 85Hertz) there is no perceptible flicker. The brain is fooled and extracts the 3D information as usual. No headaches, because the eyes are functioning as they normally do when looking at a computer.
Flicker fusion rate is high in bright areas of a picture, and flicker is best seen with averted vision on a CRT monitor. Fusion frequency in dim lights can fall as low as 10 cycles per second. Look up and see if the computer is flickering in the corner of your eye, for example in the white cartoon to the left. (CRT = Cathode Ray Tube - the traditional big glass bottle used for computer and TV screens). TFT (LCD) computer screens do not produce so much flicker, even if they are brighter, since they do not turn on and off so abruptly as CRT. Some people find TFT screens cause a burning sensation in the eyes and headaches.
The author had this problem after changing to a lovely 22 inch LCD screen at work, but discovered the cure was to avoid big white areas, such as word processing documents in black print on a white screen. Change the Windows defaults so the white background turns to a darker colour (I prefer light brown) and also change the screen font to a bigger size.
(This subject was researched in case 3D viewing on TFT screens might cause eyestrain, but the problem with LCD is unusually fine detail displayed at remarkably high contrast on a large very bright surface. This causes glare combined with eye focus problems, which are prolonged over an 8 hour working day, resulting in eye burning pains and headaches. People often screw up their faces to see the fine print, causing muscle ache or headache. Most stereoscopic pictures are not high contrast, fine detail and do not cause this problem).
Advice on prolonged computer use from Keystone company. Not quite the same advice as here, but make your own judgment. They suggest black print on a white screen is good - I disagree rather strongly about that.
Flicker fusion shows the eyes are turning on and off, which is controlled in the retina itself. Small insects have a very high fusion frequency of around 300 hertz, which is why it is hard to catch a fly, especially on a summer day when its muscles are warmed up. Our hand crashing down in a blur is seen by the fly as a large number of static images, giving plenty of time to fly off, by a direct reflex from the eye to the wing muscles, bypassing what little pathetic brain the fly might have.
Liquid crystal glasses used for 3D have caused epilepsy in susceptible people. Flickering lights can trigger an abnormal zone in the brain, where nerve impulses circulate in a loop. If the time for impulses to circle the aberrant nerve loop is close to the light flicker frequency, the feedback loop resonates and starts a seizure. CRT television, Video games and disco lights are other causes of this. Flickering below 5 hertz and over 80 cycles per second is usually OK. Big light areas cause more trouble, which is one of the reasons for not using a white background for web pages, although computer screens usually refresh so fast there is less trouble than with TV. LCD screens flicker less because they do not turn off so fast as CRT screens.
Modern stereo movies, like Avatar, have exposed the 15 to 20 percent of people with binocular fusion difficulty because they end up with:
Light normally comes from above (the sun) and our brain interprets unfamiliar scenes by assuming the light source is above. For many people, this heart shaped cavity on the surface of Mars only has its correct 3D in the middle image, otherwise the cavity turns into a hill.
This impression of 3D does not require two eyes and is called a monoscopic clue to the third dimension. There are more monoscopic clues, as we will review. It is important to combine stereoscopic clues and monoscopic clues so they work in the same direction, otherwise the brain is confused. Often a monoscopic clue will overpower the stereoscopic information, because monoscopic clues are more primitive.
Escape from 3D: New Zealand Images