Two-eyed vision is needed for depth perception. If one eye is lazy or not viewing the object at the same time as the other eye it causes a lack of depth perception. "Depth Perception" has very specific and limited meaning. This is the distance straight ahead of the viewer's eye, toward or into an object or surface. By definition, depth is looking straight into a hole or tube and estimating forward distances. Doing this accurately requires binocular stereoscopic vision (stereopsis) and may be more difficult or less accurate for people lacking stereopsis. Their depth vision must rely on visual cues other than stereopsis. http://www.abledata.com/abledata_docs/Distance_Perception.htm Ocular conditions such as amblyopia and strabismus may reduce the perception of depth. A person with only one functioning eye has absolutely no binocular depth perception. http://en.wikipedia.org/wiki/Depth_perception Amblyopia has been estimated to affect 1 to 5% of the population. Amblyopia is a developmental problem in the brain, not an organic problem in the eye. The part of the brain corresponding to the visual system from the affected eye is not stimulated properly and develops abnormally. This has been confirmed in brain specimens. Many children who have amblyopia, especially those who are only mildly amblyopic, are not even aware they have the condition until tested at older ages, since the vision in their stronger eye is normal. However, people who have severe amblyopia may experience associated vision disorder, most notably poor depth perception. http://en.wikipedia.org/wiki/Amblyopia