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.

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