Your appointment could be as brief as half an hour, but it will more likely require an hour or longer. The length of your exam depends on several factors: What is the doctor looking for? Which, among the dozens of tests available, will be used? In a routine comprehensive exam, the doctor will probably check your eyes for:
- myopia (nearsightedness; seeing near objects better than far objects)
- hyperopia (farsightedness; seeing distant objects more clearly than near ones)
- presbyopia (a form of farsightedness that begins at age forty to forty-five)
- astigmatism (irregular lens shape that distorts your vision slightly)
- strabismus ("cross-eye") and amblyopia ("lazy eye")
- glaucoma (high fluid pressure within the eye)
- cataracts, color blindness, blocked tear ducts, eye injury, defects on the cornea, and damage to the retina or optic nerve
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Visual Acuity Test
A test for visual acuity refers to the clarity of your vision. (Acuity is from the Latin acuitas, which means "sharpness.") In other words, how well do you see?
Your eye doctor will probably use the familiar eye chart to test your vision at various distances. The eye chart most of us know is called Snellen's chart, after the nineteenth-century Dutch ophthalmologist Hermann Snellen, who invented it. It consists of rows of black letters - very large at the top, very small at the bottom - against a white background.
Each eye will be tested separately while the other eye is covered. If you've had a problem with glare, the doctor will probably test your visual acuity using a variety of lighting sources,
The results of your visual acuity test are expressed by phrases such as "20/20 vision" and "20/40 vision," which some people find confusing.
The first number in the phrase, in the United States at least, is always 20 - which is the distance, in feet, you're standing from the eye chart. (Where the metric system is used, the first number is 6, indicating that the patient is standing 6 meters from the chart.)
The second number conveys how much your visual acuity differs from "normal" eyesight. If you have 20/20 vision, you can see at 20 feet what other people with good vision can see at 20 feet. If your vision is 20/40, you can see at 20 feet what people with good vision can see at 40 feet. The higher the second number, the worse your visual acuity. If your vision is 20/70 or worse, you have low vision.
At 20/200 - meaning that someone with "normal" vision standing 200 feet away can see the chart as well as you can at 20 feet away - you are considered legally blind. (Only about 10 percent of legally blind Americans have zero visual acuity; the rest have some degree of sight.)
Eye-Movement Examination and Cover Tests
There are other low-tech procedures that are probably familiar to you. These tests don't require fancy equipment, but they give the doctor a lot of important information, including whether you have cross-eye or lazy eye and how good your depth perception is.
He or she will ask you to look upward and downward, and to the right and the left. Then you'll be asked to stare at an object - first at a distance and later up close. The doctor will cover one of your eyes and quickly note how much the uncovered eye moves to adjust, then repeat the process with the other eye. He or she will probably hold an object, perhaps a pencil, near your eyes and ask you to "follow" it as it moves from side to side.
Iris and Pupil Examination
The doctor will check the appearance of the iris. Is it symmetrical? Does the pupil respond correctly to light, dilating and constricting as needed? What is the size of the pupils?
Refractive Error
If your vision is worse than 20/20, the doctor will perform a variety of tests to determine the correction needed - that is, to come up with an accurate prescription for eyeglasses or contact lenses.
The degree of farsightedness, nearsightedness, astigmatism, or presbyopia is called refractive error. To measure refractive error precisely, the doctor will probably use another rather old-fashioned device called a phoropter.
If you've had an eye exam, you're probably familiar with a phoropter. It is a complete range of corrective lenses that can be adjusted to offer you hundreds of combinations. The doctor adjusts the lenses and asks you to indicate which of two combinations is better. By continually changing the lenses, the doctor can arrive at a combination of lens strengths that will be the basis of your prescription.
Though there are automated devices for testing refractive error, many eye doctors report getting the best results by using them in conjunction with the more-subjective phoropter. An autorefractor emits a pinpoint beam of light that reflects off the retina and measures the eye's response. Autorefractors are especially useful when the patient is a small child or, perhaps, an adult who is unable to respond accurately to phoropter combinations. Some ophthalmologists use advanced computerized equipment, such as a high-tech scanner called a wavefront aberrometer, for more-detailed results. To find out more, you can check out Latest Vision Correction Techniques.
