Myopia - If your eye is longer than
usual or the cornea and lens have too much focusing power, light rays
focus in front of the retina instead of on it. This refractive error
is called myopia, or nearsightedness, and means you have a hard time
seeing objects at a distance.
Hyperopia - If your eye is shorter
than usual, or the cornea and lens lack the necessary focusing power,
light rays focus beyond the retina. This refractive error is called
hyperopia, or farsightedness, and means you can't see close objects
clearly. A young person may be mildly farsighted but unaware of it
because lens the lens is flexible and compensates for the error by
bringing the light rays forward and onto the retina.
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Astigmatism - Astigmatism occurs when the curve
of the cornea is uneven, steeper in one direction than in
the other like the back of a spoon. This uneven curve causes
light rays to focus on many points on the retina, distorting
both near and far vision.
Presbyopia - If you have presbyopia, or "aging eye," close
objects appear blurry. Presbyopia usually begins around age 40 when the lens
begins to harden and lose its elasticity. This refractive error is usually corrected
with bifocals or reading glasses. Nearsighted people with presbyopia can often
read without glasses, but farsighted people will probably need reading glasses
around age 40.
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How the Eye Works - The cornea
is the transparent tissue that covers the iris, or colored part
of
the eye. For you to see clearly the cornea and lens must bend
light rays so they focus on the retina, a layer of light-sensitive
cells lining the back of the eye. The retina converts light rays
into impulses that are sent through the optic nerve to the brain,
where they are recognized as images. If the light rays do not
focus on the retina properly, the image you see is blurry. This
problem is called a refractive error. Glasses, contacts and refractive
surgery correct or reduce the refractive error by focusing light
rays closer to or directly onto your retina.
