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Clear vision occurs when a number of factors combine to produce optical perfection: Light entering the eye through a well-formed cornea focuses precisely on the retina; the resulting image is sharp and bright.
For many of us, however, clear vision requires the corrective properties of glasses or contacts.
Nearsightedness (Myopia) occurs when the curve of the cornea is too steep, or the eye itself is too long, causing light rays to focus in front of the retina. This is the most common refractive condition and causes a visual focusing problem affecting nearly 25% of the population. The resulting image is blurry, especially when the object is far away.
Traditionally, glasses or contact lenses are used to focus the light directly on the retina. This condition can be inherited; it usually starts in childhood and stabilized in the late teens or early twenties.
Farsightedness (Hyperopia) manifests itself in a blurring of close objects, such as the print in books or on musical scores. In this instance light passing through a cornea that is too flat, or an eye that is too short, focuses behind the retina.
Astigmatism is the term for an irregularly shaped cornea (somewhat like a football), resulting in light rays focusing that focus on more than one point on the retina. This causes distortion of the object being viewed. Astigmatism is relatively common and with the astigmatic eye, the refractive elements do not focus light to a point. This occurs because the refractive surfaces cannot form a clear image of an object at any distance.
The major cause of astigmatism is usually the cornea, but astigmatism of the lens can also occur.
Presbyopia describes natural change that occurs in all eyes as we age. When you are under 40, the lens in your eye is soft and flexible. The lens of the eye changes its shape easily, allowing you to focus on objects both close and far away. After the age of 40, the lens becomes more rigid. Because the lens can't change shape as easily as it once did, it is more difficult to read at close range. This perfectly normal condition is called presbyopia.
You can also have presbyopia in combination with myopia, hyperopia or astigmatism.
Refractive surgery can partially correct presbyopia through monovision.
Emmetropia (Normal Eye) is the term for an eye with perfect vision. The light rays are focused directly on the retina.
Nearsightedness, farsightedness, and astigmatism can all be corrected in varying degrees by the Stanford Eye Laser Center.