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Monovision is the practice of seeing with different visual acuities in the right and left eyes. Because the brain is separated into right and left hemispheres, it is entirely possible to exercise independent acuities (focus) for each eye and achieve a normal vision practice. This is most often applied to a person who has presbyopia, the inability to focus on near objects after being corrected for distance. This occurs over time when a nearsighted person is constantly focusing on near objects with their corrective lenses (glasses or contacts). The natural lens of the eye is being overworked, thinning itself for distance then fattening to a convex shape for near. This constant cycle produces hysteresis in the natural lens, causing it to stiffen and lose its ability to change shape. Although unfortunate to lose binocular acuity, this can be overcome with the practice of monovision.
Ways for nearsighted people to avoid presbyopia is by slipping their distance glasses off when reading, or if using contacts, wear them for only a minimal time of up to eight hours. If doing extended close-up work, it's a good idea to take contacts out for that period, and then put them back in later.