Light rays pass through the cornea to merge and focus perfectly on the back surface of the eye, called the retina, producing clear vision.
Because the eye is longer than normal, light rays merge and focus before they reach the retina. The nearsighted eye sees close objects more clearly than distant objects
Because the eye is shorter than normal, light rays reach the retina before merging and focusing. The farsighted eye sees more clearly, but may not see any object perfectly.
Because the cornea is irregularly shaped, light rays focus at multiple points within the eye and distort vision. The astigmatic eye cannot focus clearly at any distance. Astigmatism often occurs in combination with near or farsightedness.
In the natural processes of aging, the protein composition of the lens changes, making it harder and less flexible. The lens of the eye loses some of its elasticity and therefore its ability to change focus. Beginning sometime during our mid-forties, presbyopia diminishes close-range vision.