Why is binocular vision important to humans

How can we see spatially?

To a large extent, humans perceive their environment with their eyes. He can estimate how far away things are, what position they are in relation to each other and much more. The human brain relies on properties of perspective and its own experiences.

Spatial perception is created through the interpretation of image information that is recorded with the eyes and processed in the brain. The mechanisms with which our brain derives the visual impression from the images on the retina are based on monocular (one-eyed) and binocular (two-eyed) vision. Monocular vision ensures that we already have a perception of depth when looking at a two-dimensional image - such as a photograph or when looking with only one eye. Our brain uses, among other things, experiences of seeing as well as perspective and optical effects.

Linear perspective

Monocular vision: size and position

Anyone who has ever painted a landscape could consciously or unconsciously have used the so-called linear perspective. Lines that are parallel in reality seem to converge on the horizon. If we look at the image of a road, the edges of which are becoming more and more narrow, the brain automatically interprets this as a path leading into the distance.

Another monocular effect is that objects that are close to the horizon are interpreted by our brain as being further away than those that are significantly below or above it. Therefore, the moon often appears larger when it is just above the horizon than when it can be seen higher in the sky. In the first case, our brain estimates it to be particularly far away and gives us the impression that the moon must actually be larger than an object of the same size in another location.

Moon on the horizon

For many well-known objects, we already know which of them are roughly the same size, for example trees, houses and street lamps. Since the relative size on a two-dimensional image is related to the distance between the observer and the object, the human brain compares the objects in the field of vision with one another and draws on previous experience. After that, objects of a similar size, which are also the same size in the current image, must be approximately the same distance away.

Our experience also helps us to actually perceive objects that are only partially visible in the background as hidden and not as cut off or severed. This mechanism is used, for example, in the theater with staggered backdrops to enhance the impression of depth.

Light and shadow

The incidence of light is another factor that determines spatial perception in monocular vision. Spatial bodies have a certain distribution of brightness on their surface, elevations and depressions also cast shadows. In order to distinguish inside and outside bulges without contradiction, the brain has to know the position of the light source. People prefer to accept light from above, since the sun is our natural light source. Corresponding shades on a drawing make the objects shown appear more three-dimensional.

When estimating the distance, we also use the light as a guide. Since dust particles in the air obscure the light, objects that are far away appear blurred and have a bluish tint. This effect can be seen in photographs as well as in three-dimensional reality.

Look at the bottle

Binocular vision

With binocular vision, the visual impression is composed of the slightly different images that our eyes deliver. One of the strongest effects in binocular vision is parallax. It is based on the fact that the eyes depict a close object from significantly different perspectives. This is why our thumb seems to “jump” when we hold it close to our face and look at it alternately with one eye and the other. A distant object, on the other hand, has roughly the same position when viewed from the two positions of the eyes.

The so-called convergence is still part of binocular vision. When looking at an object, the focus of the two eyes is at the same point, the two lines of sight form an angle. This angle is larger with objects in the vicinity than with objects that are far away. So that the image on the retina always remains sharp, the eye lenses usually have to curve more the closer the object is. This is done via special muscles on the lenses, the brain registers their contraction and can thus obtain information about the depth of the image.

Three-dimensional images in film

The combination of all mechanisms of monocular and binocular vision creates a spatial, three-dimensional visual impression. However, only with monocular vision, for example when looking at a photo or a classic film, full spatial impressions are not possible. In order to create a visual impression in a 3D film that is comparable to that in reality, two different images must therefore be perceived by the right and left eye. Accordingly, images are required for playback that were recorded with two optics at different points of view.