Binocular vision is defined as blending of separate images seen by each eye into one composite image. This is also called fusion. The ability to use both eyes together. In order for one to be able to appreciate depth perception or 3D, binocular vision is required. People who have severe lazy eyes, constant crossed-eyes, or one blind eye do not possess binocular vision because in most of cases only one eye (dominant) is being used at a given time, the non-dominant eye is supressed (no visual brain function). This is an adaptive measure for the brain since it is impossible for brain to fuse two very dis-similar images, therefore seeing 3D is impossible to them. For people who have misalignment of the eyes have difficulty point two eyes at one point for clear vision then they may have headach or discomfort when seeing. If a person has a problem moving the eyes inward, he will have difficulty reading when reading since eyes turn in when we view object close. Vice versa, if a person has trouble move the eyes outward, he will either see double or have headache. As an adaptive measure, these people may suppress one eye to avoid discomfort.