Stereoscopic vision

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Stereoscopic vision (from ancient Greek στερεός stereós "solid", "rigid" and σκοπεῖν skopéin "look", "observe"), also called spatial vision , stereo vision or stereopsis , conveys a real, quantifiable depth perception and through the two-eyed observation of objects and objects spatial effect of the outside space. The stereoscopic vision is the highest form of binocular vision ( binocular vision ). Seeing with only one eye is called monovision ( monocular vision ).


Spatial vision essentially requires the following prerequisites:

  • There are two eyes with coordinated muscle control and normal (or at least harmonious abnormal) retinal correspondence , a fusion allows (fusion) of the two image impressions of the right and left eyes into a single (binocular single vision).
  • The visual center of the brain processes the images obtained and models them spatially.

When looking at an object, each eye assumes a slightly different angle to it, which creates a parallax . The farther away an object is, the more the approach lines of sight of the eyes to a parallel position . If, on the other hand, one looks at very close objects, the eyes must perform a convergence movement.

This basic eye movement is usually not consciously noticed. Early learning processes allow both eyes to automatically fixate on the point that you want to look at. Many optical illusions result from the fact that the brain works based on experience and one can become confused by the fact that something does not appear as one is used to.

Research on infants by a team from the Budapest University of Technology and Economics suggests that spatial vision is a learned skill. According to this, if premature babies are exposed to visual stimuli, binocular vision can be detected in approximately the same period after birth as in babies born at the normal time.


The basis of stereoscopic vision is the imaging of objects viewed from outside within the so-called Panum area . This represents an area in front of and behind the surface of the horopter in which objects can also be easily seen binocularly that are not projected onto exactly corresponding retinal locations. This leads to a lateral disparity of the presented test objects or images, the extent of which is expressed in arc seconds . The smaller the lateral disparity, the higher the quality of spatial vision. The value of a person with normal vision is around 20 arc seconds. Differences in the examination result can arise from different measurement methods.

If the stereo image pair is swapped, the depth display is reversed and the perception corresponds to pseudoscopic vision. More distant objects are now apparently in the foreground and near object points are perceived in deeper image levels. If the stereo image pair corresponds to the position of the stereo image recording, orthoscopic vision, a spatial image corresponding to the usual spatial viewing habit, is possible. If both fields of the stereo recording are identical, no spatial perception is possible.

The cross look

The cross view (also called cross view in English) is the conscious application of the convergence mechanism in order to make a spatial image (stereo image) visible from the two partial images. In fact, we always use the cross look when we focus relatively closely , for example when reading: If we have a book or the daily newspaper in front of our noses, someone sitting across from us would have the impression that we are squinting . However, this impression has nothing to do with the actual strabismus disease.

When practicing the cross look, it is only a matter of making it clear to the brain through practice (creating experience) that what we initially see blurred is okay. We have only shifted the focus.

Anyone who is able to see things clearly at a distance of 20–60 cm has all the physiological requirements for the conscious application of the cross view. When squinting, a virtual third image is created between the two images, which offers the desired spatial impression. The recommended viewing distance for the example below is approx. 70 cm.

Myopia or farsightedness does not impair the ability to look cross-eyed as long as glasses correct the poor eyesight. However, this method leads to eye fatigue relatively quickly. In addition, the brain interprets the image as very small due to the close crossing point.

With the cross look, the puzzles with the differences between two pictures can also be solved in a simple way.

Example of differences

However, color differences are not so easy to recognize, since all spatial vision is based on shapes.

One very noticeable sign (in the example on the right) is that it is slightly shifted horizontally in both images. By comparing the two images on the other hand, it is difficult to find the difference, especially if there are large distances between the characters, so the eye has no clue.

In general, let A be the eye distance of the observer, B the distance between the stereoscopic object pairs and d the viewing distance , then the apparent image distance b is calculated using the ray theorem using the equation

The size of this virtual image results from the ratio b / d and the size of the individual images. In the example below, the distance (= the width) of the individual images is around 9 cm (depending on the screen size), so at 6.5 cm eye relief and 70 cm viewing distance, the spatial impression of a miniature only approx. 3.8 cm in size is obtained at a distance of 29 cm.

Stereo image pair for the cross view (squint method)

The parallel view

With parallel vision, one uses the other of the above-mentioned viewing techniques, relaxed looking through , to produce the spatial image. Again, many people think that they do not can . In fact, anyone who is able to see an infinite distance can. Anyone who looks at the sunset on the horizon and sees a sharp picture, who perceives the stars in the night sky as small sharp points, has all the physiological prerequisites that he needs for the application of the parallel view.

For this, however, the requirements for the double image are higher, since the distance between two objects to be overlaid must not be greater than the eye distance of the observer. That is about 6–7 cm. On the other hand, for many people, the eyes do not tire as quickly as the muscles involved are less tense. In addition, the brain interprets an image viewed in parallel as being further away and therefore larger. The parallel view is therefore better suited for panoramic images. An extension of the method by means of suitable hardware is used, among other things, in the 3D representation of virtual reality .

The apparent image distance b results from the distance B of the individual images, the eye distance A and the viewing distance d :

The case B = A corresponds to an infinitely large distance and thus the stereoscopic impression of a distant object. For greater distances between the individual images, b becomes formally negative; to the viewer, however, if the individual images are superimposed, it appears as if the object is even further away than with normal distant vision. In this case, however, the eyes can also tire quickly.

One method to use the parallel view is the following:
First, fix an object approx. 1 to 3 m away. Then you push the stereogram into your field of vision, but without fixing your eyes on this closer object. Slowly parts of the picture stand out and you begin to recognize something.

Hyperscope and pseudoscope

Hyperscope and pseudoscope

The hyperscope optically increases the eye relief, which increases the impression of spatial depth. Some distance measuring devices work according to this principle, but so does the scissor telescope .

The pseudoscope optically swaps the positions of the eyes, which reverses the impression of spatial depth.


Different ametropia in both eyes ( anisometropia ) can impair three-dimensional vision, as in this case the brain has to merge two images of different sizes ( aniseikonia ) into one. A spectacle , the optical defects correct though, but because their lenses have a certain distance from the eye itself, the image in the eye either increased ( farsightedness ) or decreased ( nearsightedness ). This hampers stereoscopic vision especially when the refractive ratios of the two eyes show major differences. In this case, contact lenses are preferable to glasses.

If a person crosses , three-dimensional vision is also often not possible, since the visual impression of the cross-eyed eye is suppressed in most congenital, manifest diseases of the squint. In acquired strabismus diseases, the prognosis for maintaining or regaining spatial vision through suitable treatment measures (e.g. strabismus surgery or prism glasses ) is significantly better.

If only one eye is used - regardless of whether there is only one or because one eye is excluded from seeing because of a squint - spatial vision is also impossible.

Spatial vision cannot be replaced in terms of its quality, but its lack can be compensated for by certain phenomena - e.g. B. through perspective, light and shadow, empirical values, movement, etc. For this reason, one-eyed people - or even other people without spatial vision - are allowed to drive a car. However, this also has limits, namely where a perfect stereopsis is essential - e.g. B. with truck, bus or taxi drivers, pilots etc.

The complete lack or complete loss of spatial vision is also known as stereo blindness , especially in the Anglo-Saxon-speaking world .

Web links

Individual evidence

  1. Jump up Gábor Jandó, Eszter Mikó-Baráth, Katalin Markó, Katalin Hollódy, Béla Török & Ilona Kovacs: Early-onset binocularity in preterm infants reveals experience-dependent visual development in humans. In: Proceedings of the National Academy of Sciences . June 18, 2012, doi: 10.1073 / pnas.1203096109
  2. Brain research: Spatial vision has to be learned . In: Spiegel Online . June 19, 2012