Refractive Error
Though some people who wear bifocals do so only to correct presbyopia, most bifocal wearers also have refractive errors that affect their distance vision.
Refractive errors are imperfections in how the eye focuses light. In most cases, uncorrected refractive errors cause noticeably blurred vision.
In an eye with perfect vision, light is focused precisely on the retina, which is the light sensitive inner lining of the back of the eye. In eyes with refractive errors, the power of the focusing components of the eye (the cornea and lens) are too strong, too weak or asymmetrical, so light dos not come to a focal point on the retina.
There are three main refractive errors are nearsightedness, farsightedness and astigmatism.
These common refractive errors can be corrected with prescription eyeglasses, contact lenses and LASIK or other refractive surgery.
The degree of refractive errors and the power of lenses to correct them are measured in units called diopters (D).
Nearsightedness
In nearsightedness (also called myopia), the eye focuses light too quickly, before it reaches the retina. Myopia causes blurred distance vision; near vision remains clear (hence, you are "near-sighted").
According to the National Health and Nutrition Examination Survey conducted in 1999-2004, approximately 41.6 percent of Americans are nearsighted.
Lenses that correct myopia are called minus (-) lenses. These lenses move the focus point of the eye backward, onto the retina.
Farsightedness
In farsightedness (also called hyperopia), the eye is not powerful enough to bring light to a focal point on the retina. Farsightedness usually causes only near vision to be blurred (you remain "far-sighted"), but higher amounts of hyperopia can cause blurry vision at all distances.
In mild cases of farsightedness, the primary symptoms may be eye fatigue and headaches when reading or performing other near vision tasks, rather than blurred vision.
According to the Baltimore Eye Survey conducted in 1991, approximately 45 percent of Americans over age 40 have at least 0.75 D of farsightedness.
Lenses that correct farsightedness are called plus (+) lenses. These lenses add focusing power to the eye so light can be focused on the retina for clear, comfortable vision.
Farsightedness sometimes is confused with presbyopia, which is a normal age-related loss of near vision that occurs after age 40. Virtually everyone, including farsighted individuals and people with good vision throughout most of their life, will experience presbyopia when they get older.
Presbyopia is treated with bifocals, trifocals and progressive multifocal lenses if a person also has refractive errors. Most people who have good distance vision without glasses choose reading glasses when they become presbyopic.
Astigmatism
Astigmatism (sometimes wrongly called "stigmatism") occurs when the front surface of the eye (cornea) and/or the lens inside the eye has an asymmetrical curvature, which prevents light from being focused to a single point on the retina. Astigmatism causes blurred vision at all distances.
Astigmatism can occur along with nearsightedness or farsightedness. Studies suggest approximately 30 percent of people worldwide are affected by astigmatism.
An eyeglasses or contact lens prescription for astigmatism has two powers: a sphere power and a cylinder power. The sphere power corrects myopia or hyperopia. The cylinder power corrects the astigmatism.
Lens prescriptions for astigmatism also include an "axis" designation. This is a number between 1 and 180 that identifies the meridian of the cylinder power in the lens.
Astigmatism generally is stable throughout a person's lifetime. However, certain eye conditions can cause a significant increase in astigmatism.
Keratoconus is an eye disease that causes progressively higher amounts of irregular astigmatism, due to thinning and bulging of the cornea. New keratoconus treatment options such as corneal collagen cross-linking (CXL) are showing promise in stabilizing the condition and restoring good vision.
Cataracts also can cause changes in astigmatism.
Eye Exams
A comprehensive eye exam performed by an optometrist or ophthalmologist is the best way to determine if you have a refractive error. During the exam, your doctor will perform an eye test called a refraction to see if you have nearsightedness, farsightedness or astigmatism that requires correction.
If a refractive error is diagnosed, ask your eye doctor to discuss all treatment options to determine whether eyeglasses, contact lenses or refractive surgery is the best choice for you.
Preventing or Reversing Refractive Errors
Some companies advertise programs of eye exercises that they say can prevent or reverse refractive errors. Most eye care professionals agree that these "treatments" are not effective in correcting or reversing nearsightedness, farsightedness and astigmatism.
These programs should not be confused with legitimate vision therapy programs that are used to correct binocular vision problems such as poor eye teaming, focusing flexibility or depth perception.
Before considering any program of eye exercises, consult your eye doctor.