The situations presented so far represent normal vision throughout a person's lifetime. One problem could be that the optical system simply does not have enough optical power to converge parallel rays onto the retina. This condition is called hyperopia or farsightedness. Eyeglasses come to the rescue. The simple fix is to place a convex lens (positive diopter) in front of the eye, as in the case of reading glasses. In the opposite direction, some eyes have too much optical power. This case is called myopia or nearsightedness, and a concave lens (negative diopter) is placed in front of the eye to reduce the optical power appropriately. Recall that we have two eyes, not one. This allows the possibility for each eye to have a different problem, resulting in different lens diopters per eye. Other vision problems may exist beyond optical power. The most common is astigmatism, which was covered in Section 4.3. In human eyes this is caused by the cornea having an excessively elliptical shape, rather than being radially symmetric. Special, non-simple lenses are needed to correct this condition. You might also wonder whether the aberrations from Section 4.3, such as chromatic aberration, occur in the human eye. They do, however they are corrected automatically by our brains because we have learned to interpret such flawed images our entire lives!
Steven M LaValle 2016-12-31