Eyesight Optics Modeled very simply, the human eye consists of a variable focal length convex lens that is intended to adjust itself so that, regardless of the object distance, the image distance remains about 2.5cm in order to project a clear image onto the retina at the back of the eyeball. The curved shape of the cornea at the front of the eyeball does most of the focusing for vision, but it cannot change shape and so the lens is critical to seeing objects clearly at various distances. For all practical purposes, the two extremes in object distance are 0.25m (the near-point) and infinity (the far-point). Children can often focus on something much closer than 25cm from the eye without discomfort, but as we age, both the maximum distance we can see and the minimum at which we can bring objects into sharp focus both become restricted. A near-sighted (myopic) person has a far-point reduced from infinity to some smaller distance beyond which objects appear problematically fuzzy. A severely near-sighted person may have a far-point of as little as a meter. The purpose of corrective lenses in this
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