The 3D sensation is a reconstruction of space by our brain, from several informations that it analyses. These informations are: 

  • the shift of two images coming from our left eye and from our right eye.
  • the convergence of our two eyes when our sight is aiming a particular subject.
  • the accommodation, which is the focusing of our eye on a near object.
  • the disposition of plans: a tree is hiding a house, thus it is in the front.
  • the lighting which make us to know the hollows and the bumps.
  • our knowledge, on the true size of objects and things: a dog is smaller than a man, so, if it looks so important, this is because it is much near, etc...
 Of course, each of these informations has its own limits.


Thus, as each case, some datas will prevail, and other less, or will not. For the near subjects , there will be the datas of image shift, of convergence, and  accommodation which will be predominant. For the subjects at intermediate distance, these will be the convergence, the arrangement, and the lighting. Whereas these will be mainly the arrangement, the lighting, and our knowledge.
Why that? Because our eyes have functioning physic limitations, and our visual keenness, our ability to accommodate, or to converge are only efficient within some domains.
The keenness allows us to distinguish two different dots if they are apart of more than one minute of angle.
The accommodation of our eyes let us clearly distinguish an object located at some tenth centimeters when we are young, and at only 2 meters when we are older. Beyond 3m, the accommodation is not necessary.
The convergence is necessary to fix an object until a mere 40m, beyond 50m, it is not necessary.
Windmills of Antoinette and Jean-Claude Anquier, Berck 2011
crossed eye method