The eye functions much like a camera. Light passes through the cornea and the lens of the eye and is focused on the retina in much the same way that an image is focused on the film of a camera.

Click here to see an example of how 20/20 compares to other visual acuities.

The cornea and the lens are transparent and bend light rays in such a way that an image is projected onto the the retina. The iris, the colored part of eye, contains the pupil which controls the amount of light entering the eye in much the same way as the shutter of a camera controls the amount of light entering a camera. The light rays then pass through the vitreous cavity that is filled with a clear gelatinous material and are finally focused on the retina. The image on the retina is then transmitted to the brain through the optic nerve

This page gives you the various natural conditions of the eye unrelated to diseases of the eye.
  
Note:  Be sure to slide your mouse over each of the "eye" graphics to see the image change and offer you additional information.

Myopia (Nearsightedness)
 
Normal vision - to see clearly, light rays are focused on the retina by both the cornea and the lens of the eye. In a myopic eye, commonly referred to as "nearsighted", light rays from objects are focused in front of the retina causing images - particularly at a distance - to be blurred.

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Hyperopia (Farsightedness)
 
Normal vision - to see clearly, light rays are focused on the retina by both the cornea and the lens of the eye. In a hyperopic eye, commonly referred to as "farsighted", light rays from objects are focused in back of the retina causing images - particularly close up - to be blurred. This is not to be confused with Presbyopia.

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Astigmatism

Normal vision - light traveling through the cornea in both the horizontal plane (blue) and vertical plane (red) is refracted (bent) the same, so that the image is properly focused on the retina in the back of the eye. Vision is clear. With astigmatism, an abnormal curvature of the cornea in one axis, the horizontal axis (blue) in this example, causes light traveling in this plane to focus in front of the retina. The light reaching the retina in this plane is out of focus and vision is unclear.

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Accomodation

Normally, when close objects are viewed, such as a book, the lens of the eye (yellow arrow) will change shape to cause light to focus on the retina (blue arrow). Close vision is clear.

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Presbyopia

Presbyopia is the normal process of aging, The natural lens in your eye (yellow arrow) continues to add layers as we grow older. This causes the lens to become larger. As the lens grows, it enlarges into the area where the focusing muscle needs to flex when it works. We become unable to focus things up close because of this loss of room around the lens. This usually occurs between the ages of 40 and 50. Everyone experiences presbyopia, some sooner, some later. Because of this normal process, nearsighted people begin to wear bifocals in their forties, and those who never needed glasses before may require reading glasses. Reading glasses can correct presbyopia by allowing light to focus properly on the retina (blue arrow).

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