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Kornmehl Laser Eye Associates

Laser Vision Correction, Dry Eye Treatment, Cataract Surgery,
Contact Lenses & General Eyecare

How the Human Eye Works Part 2

Common Vision Problems

Your Boston eye doctor may refer to your vision problem as your refractive error, or focusing problem.  How well you see is determined, for the most part, by how accurately your eyes are able to bend, or refract, light.  In a normal eye, the focus comes to a point on the retina.  But sometimes this does not occur.  The result?  Various forms of vision impairment, or aberrations.  Vision problems fall into one of two basic groups: low-order aberrations and higher-order aberrations.

Low order-aberrations, also known as nearsightedness, myopia is a condition in which you can see nearby objects well, but objects at a distance appear blurred.  This happens when light bouncing off a faraway image enters the eye through the cornea and comes to a point of focus too soon, before it reaches the retina.  Myopia may be due to a cornea that has too much curvature, which causes the light to “overbend” and focus in front of the retina.  Myopia also occurs when the eyeball is too long- the retinal wall is too far back for the combined focusing power of the cornea and lens.

People with hyperopia, or farsightedness, see distant objects more clearly than nearby objects when they are young but may have difficulty with both as they get older.  In hyperopia, the light rays coming into the cornea are not bent sharply enough and are focused behind, rather than on, the retina.  The result is a blurred image.  This usually happens in people whose eyeballs are too short from front to back or whose focusing muscles around the lens are too weak.  Another cause of hyperopia, though rare, is a cornea that is not curved enough.

Because muscles are more elastic in youth, younger people who are mildly hyperopic can actually compensate for it by using the focusing muscles around the lens to fine-tune the focus by bending light more steeply.  This action brings the point of focus forward toward the retina, allowing them to see more clearly.  However, because the muscles weaken and the lens becomes less pliable as we age, the individuals eventually lose the ability and may no longer wee well at a distance or close up.  After age forty, they may be completely dependent on eyeglasses or contact lenses for both close and distant vision.

Many individuals with myopia or hyperopia also have some degree of astigmatism.  People with significant astigmatism experience blurred or distorted vision with all objects, whether near or far.  Astigmatism means that your cornea, instead of being spherical like the side of a basketball, is slightly oval, shaped more like the side of a football.  Your cornea is more curved in one direction that the other.  As a result, light rays entering the eye from different points on the cornea’s surface are bent irregularly and are focused at several different points, rather than meeting at just one focal point.  Almost everyone has a small degree of astigmatism.

Farsightedness is often confused with presbyopia, which literally means “old eyes”.  Presbyopia is the age-dependent need for reading glasses or bifocals.  After age forty, and in most people by age forty-five, the ability to focus on an object close up, such as a restaurant menu, becomes more difficult.  This happens to everyone.  It is due to a loss of flexibility in the lens and a weakening in the muscles that enable the lens to flex and fin-tune the focus.  Presbyopia  typically continues to worsen until age sixty-five.  When this occurs, people who already wear eyeglasses may need bifocals, and those who have never worn eyeglasses may require reading glasses.

Higher-order aberrations are focusing problems that are not correctable with glasses or contact lenses.  Higher-order aberrations, which are a result of subtle irregularities in the focusing mechanism of the eye, cause a loss of crispness, clarity, and contrast.  If you have significant higher-order aberrations, you may have trouble distinguishing between shades of gray.  Higher-order aberrations may also affect one’s night vision; people with these problems may see glare or halos around lights.  Approximately 17 percent of visual errors are considered higher-order aberrations.