Vision is triggered when light is focused on the retina, the optic nerve is stimulated and the information is sent to the brain where it is processed and interpreted. The eye has a refractive system, which consists of the cornea (clear front surface of the eye), the lens and the retina. The refractive system’s function is to focus light onto the retina. When the refractive system does not focus light passing through the eye directly onto the retina, the result is blurry vision. This is called refractive error.

In an eye with no refractive error, light passes first through the cornea, then the lens and is finally focused to a single point on the retina, producing a clear image. In an eye with a refractive error (nearsightedness, farsightedness or astigmatism), the cornea is shaped improperly and fails to focus light passing through it to a single point on the retina. The result is blurry vision.

Normal Vision
	Good Vision is possible when light enters the eye through the cornea (clear front surface of the eye) passes through the lens and is focused at one point directly on the retina.
Myopia (Nearsightedness)
	Myopia (Nearsightedness) occurs when the eyeball is too long or the cornea (clear front surface of the eye) is too steep. People with myopia are unable to see clearly at distance although near vision will appear clear.  Glasses or contacts lenses can change the focus of light so distance vision will be clear.
Hyperopia (Farsightedness)
	Hyperopia (Farsightedness) occurs when the eyeball is too short or the cornea is too flat.  People with hyperopia are able to see at a distance although near vision is blurred. Glasses or contacts lenses can change the focus of light so near vision will be clear.
Astigmatism
	Astigmatism often accompanies nearsightedness or farsightedness.  Astigmatism occurs when the curvature of the cornea is not the same in all directions.  A cornea without astigmatism is shaped like a sphere or round like a baseball but a cornea with astigmatism has a shape similar to a football or back of a spoon with the curvature being steeper in one direction and less steep in the other. Light is focused at two different points on the retina.  Patients with astigmatism have blurry vision in both near and distance.
Presbyopia - Glasses for Near Vision (After Age 40)
	As the body matures, the lens of the eye loses its flexibility.  This loss of flexibility is also a loss of the ability to focus for near vision.  This is called presbyopia and it is a part of the normal aging process.  People who have never worn glasses find themselves needing magnifying glasses to read and do close work.  People who wear corrective lenses for myopia, hyperopia, or astigmatism need additional correction for reading.  Most people begin wearing bifocals in their forties although there are other options. Nearsighted people find they can take their glasses off for near once they experience presbyopia and this compensates for their near vision problems.  Contact lens wearers need reading glasses worn over their contact lenses for near vision or they can have their contact prescription changes to monovision once they begin to experience presbyopia.  With monovision, one eye is corrected perfectly for distance while the other eye is undercorrected so that good near vision is still possible.