In eyeglasses, prisms are used primarily for double vision, positional correction, or convergence correction.Double vision (diplopia) is when a viewer sees two individual images using both eyes, instead of the one most people see, merged together by the brain. Some people with diplopia see only a single image, but it is blurred. Others, when using only one eye, do not see in 3-D, but a single, flat image. This is the result of one eye needing positional correction. If both eyes are affected (not working in unison), a misconvergence occurs, meaning the image seen by each eye needs to be repositioned individually in order to become a single one. Sometimes, diplopia is even caused by wearing eyeglasses, and adding prisms can correct this.Prism eyeglasses are enough to correct these abnormalities if the person has nearly normal vision in their level of nearsightedness or farsightedness. However, if one of the eyes has greatly reduced vision or blindness, there could be physical or neurological issues causing the diplopia. This would then require surgery to repair the eye muscles in addition to the use of the corrective prism lenses.
Prism can be used to correct vision for an individual whose eyes are not perfectly aligned as with, for example, a patient with strabismus. When the eyes are not aligned, the right and left eye see different images resulting in blurred or double vision. Sometimes the brain can even "shut off" one eye, in an attempt to remedy the vision, resulting in monocular vision and loss of depth perception. Prism can often be used to align the images seen by both eyes, so the eyes can fuse or see the same image, restoring visual clarity and depth perception.
Prism eyeglasses are not normal glasses. They are not designed to improve the vision of one or both eyes, and they do not help a person see better at a distance or close up. They are used when an individual is having trouble keeping his eyes working together. In other words, the eyes are looking at two different points instead of the same point. Generally this will cause double vision, if the eyes are extremely bad, or blurred vision if they are looking close to the same point. An eye doctor is needed to diagnose the issue, and prism glasses are the normal remedy.