Defects
Many people suffer from errors in refraction. Myopia, or nearsightedness, causes distant
objects to appear
blurred, while the individual with hyperopia, or farsightedness, has difficulty focusing for near objects. Astigmatism causes only some parts of an object to be in focus. In presbyopia, a condition that arises in later life, the lens of the eye loses its ability to change shape and cannot focus well for near objects. Nystagmus is an involuntary repetitive to-and-fro oscillation of the eyes, and strabismus denotes misalignment of the eyes that may cause double vision and preclude the ability of the eyes to simultaneously view an object. Children with strabismus may also have amblyopia, weak vision in an eye that is normal; it is treated by patching the strong eye to force the weak one to work.
Photophobia denotes excessive sensitivity to light. It may result from certain eye diseases or disorders of the central nervous system. Color
blindness is a difficulty in recognizing one or more colors; total absence of color vision is very uncommon. Most color-vision defects are genetic, although some can be acquired as a result of certain diseases or medications. Night blindness is the inability to see when illumination is dim. This condition can occur as a result of
vitamin-A deficiency or, more commonly in developed countries where vitamin deficiency is rare, as a hereditary disorder.