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Man has the ability to physically perceive depth and thus to be able to interprete three-dimensional arrangement of objects and movements in a spatial environment. In the course of evolution this ability must have been a great advantage for human beings to move, orient themselves and maintain themselves on earth. As a consequence of the position of both eyes next to each other, the retinas of the left and right eye perceive a largely overlapping image of the world, that however, shows some shift and a slight rotation angle with respect to each other: retinal disparity. This phenomenon is called parallax. In the visual cortex of the brain these two different images are combined to one unit. A sensation of depth vision arises (more about binocular vision = vision with two eyes). In fact we do not deal with full 3D vision, since the field of vision is relatively narrow. Another restriction is that the perspective is determined by the place where the observer stays. According to the "Optometrics network" at least 12% of people have some type of problem with their binocular vision. Remarkedly, with only one eye it is often still possible to make a good estimation of the spatial distribution of objects around us (this is also possible with other senses, by the way!): we can scan the surrounding by directing our eyes and head, while our eyes stepless focus on the shapes on which we concentrate. From this bulk of visual information coupled to earlier experiences, in particular distortions related to perspective, the size and significance of objects, we are able to re-create a "depth picture" in our head.
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