Refractive Errors
The primary reason people wear eyeglasses is to correct refractive errors.
Myopia, hyperopia and astigmatism are the three main types of refractive errors.
Refractive errors usually are corrected with eyeglasses or contact lenses, but many people these days are choosing LASIK or other refractive surgery to correct their vision.
Refractive errors and the power of lenses or surgical treatments used to correct them are measured in units called diopters (D).
Myopia
Myopia (also called nearsightedness) is blurred distance vision. This occurs because the eye focuses light too quickly, before it reaches the light-sensitive retina in the back of the eye.
Slightly more than 40 percent of the U.S. population is nearsighted, according to the recent National Health and Nutrition Examination Survey.
Lenses prescribed to correct myopia are called minus (-) lenses. They move the focus point of the eye backward, onto the retina.
Hyperopia
Hyperopia (also called farsightedness) usually causes blurred near vision. But higher amounts of hyperopia can cause blurred vision at all distances.
Individuals with mild farsightedness, especially in young people, might not notice blurred vision, but instead might experience headaches and eye strain when reading or doing other near tasks.
According to a recent large population study conducted in Baltimore, 45 percent of Americans over age 40 have at least 0.75 D of farsightedness.
Lenses prescribed to correct farsightedness are called plus (+) lenses. They lenses add focusing power to the eye so light can be focused on the retina for clear, comfortable vision.
Hyperopia sometimes is confused with presbyopia, which is a normal age-related change in the eye that causes blurred near vision. The two conditions are different, and a person who is farsighted also will develop presbyopia later in life.
Hyperopia is corrected with single vision (monofocal) lenses; presbyopia is corrected with bifocals (or other multifocal lenses) or reading glasses.
Astigmatism
Astigmatism occurs when the front surface of the eye (cornea) and/or the lens inside the eye has asymmetric curves, preventing light from being focused to a single point on the retina. Because of this, astigmatism causes blurred vision at all distances.
Sometimes, astigmatism is incorrectly called "stigmatism."
Astigmatism can occur alone or combined with nearsightedness or farsightedness. It's been estimated that approximately 30 percent of the U.S. population has some degree of astigmatism.
A prescription for astigmatism includes two powers: a sphere power and a cylinder power. The sphere power corrects myopia or hyperopia; the cylinder power corrects astigmatism. The prescription also includes an "axis" designation, which is a number between 1 and 180 that identifies the location of the cylinder power in the lens.
Astigmatism usually occurs early in life and remains relatively stable. But certain eye conditions can cause a significant increase in astigmatism.
Keratoconus is an eye disease that causes the cornea to become thinner and bulge forward, causing high amounts of irregular astigmatism. In the past, the only keratoconus treatment options were gas permeable contact lenses or a corneal transplant. Today, new options such as corneal collagen cross-linking (CXL) are showing promise in stabilizing the condition.
The development of cataracts also can cause changes in astigmatism.
Eye Exams
Routine eye exams are the best way to determine if you have refractive errors or changes in your vision. During the exam, your optometrist or ophthalmologist will perform an eye test called a refraction to see if you have any myopia, hyperopia or astigmatism.
If a refractive error is diagnosed, your eye doctor can help you decide which treatment option — eyeglasses, contact lenses or refractive surgery — is best for you.
Eye Exercises
Most eye care professionals agree that eye exercises advertised on television and the Internet do not correct or prevent refractive errors. These exercises should not be confused with legitimate vision therapy prescribed by an eye doctor to correct binocular vision problems such as poor eye teaming, focusing flexibility or depth perception.
Before spending money on eye exercises, consult your eye doctor for details.