Thousands of years ago, the “accepted” eye test was to stare up into the night sky at the Big Dipper. You were looking for Alcor, the second star in the handle. Right next to it—in astronomical terms, at least—is its optical double, Mizar. If you could see them as two separate stars, your vision was considered to be adequate.
Today, instead of looking up at a pair of stars 88 light-years away (that’s 528,000,000,000,000 miles, give or take a trillion or so), we use an eye chart 20 feet away. Although it lacks the drama of staring up into the night skies, the modern method is more convenient. It doesn’t require you to miss any of your late-night TV shows, and you don’t have to worry about air pollution reducing your visibility.
It’s also at least a smidgen more accurate.
Accuracy is a lot more important today than it was thousands of years ago, because now if the eye doctor finds something wrong with your vision, something can be done to correct it.
But first the doctor has to determine precisely how good your vision is.
Charting Distance Vision
If you have 20/20 vision, you have average or normal vision—not perfect vision mind you, just average vision. The 20/20 designation simply means that you can see at 20 feet what the average person can see at 20 feet. Many people have vision better than 20/20.
To determine what your numbers are, most eye doctors use the Snellen chart. It has lines of letters on it that start out large at the top and end up small at the bottom. The 20/20 line is near the bottom of the chart.
The chart is usually placed 20 feet away from you, and if you can read the 20/20 line, you are considered to have good vision.
But let’s suppose that you can’t read the letters on the 20/20 line, but you can make out the letters on the 20/40 line, which is three lines above it. That means that you can see at 20 feet what those with normal vision can see at 40 feet. You have 20/40 vision.
Or let’s assume your vision is very sharp and you can make out the 20/15 line, which is even smaller than the 20/20 line. That means you can see at 20 feet what someone with normal vision sees at 15 feet. You have 20/15 vision.
What if all you could see was the large E on top of the Snellen chart? Then your vision would be 20/400. You’d be seeing at 20 feet what a person with normal vision can see at 400.
Thanks to today’s corrective lens technology, having less than 20/20 vision is rarely a major problem. The problems begin if and when the vision cannot be corrected to 20/20 with glasses or contact lenses. There are people, for example, who can barely read that 20/400 Snellen E with their naked eyes, but once they put on their glasses or contact lenses they can pass the eye test for a driver’s license. They may be blind without glasses, but not with them. The legal definition of blindness, by the way, is vision that cannot be corrected to better than 20/200.
Can You Read the Fine Print?
Near vision is tested separately using a printed card, known as a Jaeger card, with rows of small type on it. The card is held 14 inches away from the eyes and the patient is asked to read the smallest line possible. Being able to read J2 or J3 means you can read most fine print. A patient who can read the J1 line—which is in smaller type than a telephone book—has great near vision.
While there are people who can read even smaller type, it’s not really worth redesigning the Jaeger card just to measure their ability. After all, you are at the eye doctor’s to find out if anything is wrong with your eyes, and if so, to get it corrected. The doctor isn’t going to do anything for clear vision—whether it be good, great, or fantastic—except congratulate you and give you some hints on keeping it that way.
One further point about near vision. Once you enter your forties, you will probably need reading glasses. Unless, of course, you are nearsighted. Some nearsighted people can still read fine print as they get older, although they sometimes have to take off their regular glasses to do it.
A Study in Contrasts
A growing number of doctors are also testing for visual contrast sensitivity using the Vision Contrast Test System (VCTS).
Contrast sensitivity refers to your ability to see the difference between two objects. Let’s say, for example, that you can read the 20/20 line—against a white background. But what if the background were gray? What if the letters were gray, too, but a different shade? And while you can see a black-and-white street sign on a bright, sunny day, can you see it on a cloudy day? Can you see it at dusk?
While the Snellen chart is filled with letters or symbols, the VCTS chart is composed of gray circles. Some circles are solid gray. Others have bars inside printed in different shades of gray. It’s up to the person being tested to say whether there’s a bar in the circle, and, if so, which way it points.
Testing for contrast sensitivity can also help the doctor spot problems such as amblyopia and more dangerous conditions such as glaucoma and cataracts at an early stage.
A Test of a Different Color
While the VCTS chart measures your ability to distinguish among varying shades of gray, color vision–testing cards test your ability to see anything other than gray.
While some people become color-blind as a result of injuries or degenerative diseases that damage the retina or optic nerve, most color-blind people are born that way. And almost all of those people are men. Approximately 8 percent of all men are color-blind to some extent, whereas less than 0.5 percent of women suffer from the condition.
Being color-blind doesn’t always mean that you’re watching the world on a black-and-white TV. Usually, color-blind people simply cannot always distinguish certain colors, especially red, green, brown, or gray.
Like the VCTS chart, the standard color vision test consists of a series of circles with numbers or objects inside them. The figures are printed in different colors and may or may not be apparent to the person taking the test.
There is no cure for color blindness, and people who are afflicted with it usually adapt to life quite easily. While they might not know if that flashing light on the road ahead is red, blue, or yellow, they do know that a flashing light means that there is some trouble ahead and that they should drive accordingly. And while they might not be able to tell if the signal light shows red, yellow, or green, they know that the top light means stop, the bottom one means go, and the one in the middle means this is their last chance to go before the stoplight comes on.
Eye Exam Guidelines
Most eye tests are conducted in an eye doctor’s office by an eye doctor or by an ophthalmic assistant.
A century or so ago, however, they were usually done by a jeweler in his back room. After all, while it took a lens maker to grind corrective lenses, it took a jeweler to put those lenses into a pair of eyeglass frames that actually fit your face.
But because ophthalmologists and optometrists have long since taken over the field from the jewelers, let’s look at what one national organization recommends as part of a complete eye exam.
- A review of the patient’s and family’s general health and eye health history (because some eye conditions are hereditary)
- An examination of the eye’s interior and exterior for signs of eye disease or other general health problems that may show up in the eyes
- A test of ability to see sharply and clearly at near and far distances
- Tests for nearsightedness, farsightedness, astigmatism, and presbyopia
- A check of eye coordination and eye muscle function to be certain the eyes are working together as a team
- A test of the ability to change focus quickly, from far to near and back again
- A glaucoma test
Special Tests for Children
Children should have their first eye exam before they are three and should have all of the above tests plus the following ones:
- A check for indications of crossed eyes or indication that the child is not using one eye
- A depth perception test
- A color vision test
- A motor skills test to check eye-hand-foot coordination
- A test to determine the child’s dominant eye
- When indicated by the child’s development, behavior, or health history, a series of tests should be conducted to determine whether or not the child’s vision skills are developing normally. Such tests may include observation of visual abilities while the child is building with blocks, copying forms on paper, or completing incomplete pictures drawn on paper. These tests are selected on the basis of the child’s age level
Help from High-Tech Equipment
Once upon a time, the only way you could find out which lenses were right for you was to put them on and look through them. And if you had to try on 100 or more different lenses or combinations of lenses to find the right ones … well, that’s life.
Fortunately, the Phoropter simplified the process.
Once an eye doctor has determined that you do need glasses—or contact lenses—the next step is this remarkable machine. It contains more than 100 different lenses in a rotary bank, and the doctor can dial different corrective strengths while asking you: “Which lens lets you see better, A or B? Okay. Now which lens is better, A or B,” and so on. Eventually you get down to two choices that give you vision so good that you yourself can’t tell the difference.
This process is referred to as bracketing. Each decision you make gets you closer and closer to the correct lens. Bracketing is merely a trial-and-error process played by the rules of logic, common sense, and good orderly procedure.
Now using the Phoropter to find the right lens is fine if the patient is capable of understanding what it is that’s going on and that it requires making a decision. But what do you do with small children?
The simplest method is to use a retinoscope. This device shines a light through the pupil of the eye that is reflected back from the retina. The way the light is reflected back tells the eye doctor what sort of lens is needed. This type of objective measuring system can be used on children or anyone else who can’t communicate. It even works with dogs or other animals.
Many doctors also use the retinoscope with all their patients before making the initial Phoropter settings, because it gives them a good idea of what range of lenses to start the bracketing procedure with.
When to See an Ophthalmologist
While optometrists provide a good deal of primary eye care, some conditions require the sort of medical examination, evaluation, and treatment that only an ophthalmologist, a medical doctor who specializes in eye care, can offer. These are the signs to look for:
- Blurry vision that can’t be corrected by lenses
- Double vision
- Sudden loss of vision
- Dimming of vision that fades in and out
- Red eye
- Eye pain
- Loss of peripheral or side vision
- Halos (colored rays or circles around lights)
- Crossed, turned, or wandering eye
- Twitching of the eyes or eyelids
- Flashes or streaks of light
- New floaters (spots, strings, or shadows in the visual field)
- Discharge, crusting, or excessive tearing
- Swelling of any part of the eye
- Droopy eyelids
- Bulging of one or both eyes
- Difference in the apparent size of the eyes or in pupil size
More and more eye doctors are investing in an automatic type of retinoscope called an autorefractor. By sending out laser beams and measuring how they are reflected back from the retina, the machine automatically finds the right prescription and prints it out. But while it can come close and is especially helpful for dealing with small children, it’s not always 100 percent accurate. Most doctors who use an autorefractor put their patients on the Phoropter afterward to fine-tune and double-check the lens selection.
Making the autorefractor part of the normal eye examination routine can shorten an average eye exam by 5 or 10 minutes. This saves time for everyone and lets the doctor see more patients.
Now let’s look at some of the other test equipment an eye doctor might use during an eye test.
A keratometer measures the curvature of the cornea. While used primarily for determining what contact lenses a person needs, it is also used after cataract and cornea transplant operations to monitor astigmatism.
A slit lamp is a large microscope that lets a doctor examine the front of the eye and look for corneal problems, infections, and cataracts.
An Eye Exam for Believers Only
When the noted nineteenth-century British biologist Thomas H. Huxley said that “a little knowledge is dangerous,” he might have been talking about Agnus Peczely. It was Peczely, an obscure Hungarian physician, who first enunciated the basic principles of iridology over a century ago.
Peczely said that almost all bodily diseases could be diagnosed by studying the minute details of the iris of the eye. The concept does make more sense than trying to foretell the future by studying the entrails of a sacrificial chicken, but not much.
It is true that some diseases, such as diabetes, high blood pressure, and atherosclerosis, can be detected by examining the eye. But Peczely took this interesting bit of diagnostic trivia and tried to base a new science on it, the pseudoscience of iridology. It made about as much sense as diagnosing character traits through phrenology, the study of the shape of bumps on a person’s head, which was also popularized in the nineteenth century.
In spite of the absurdity of these examinations, a cult of ocular diagnosticians began to flourish, particularly in central Europe. Early in this century, iridology was introduced in the United States, and it has been with us ever since. It is now primarily, but not exclusively, within the domain of a small group of chiropractors. Magazine articles, television programs, and newspaper articles have heightened public awareness of this supposed diagnostic method. An attempt to achieve further respectability for iridology was made by wrapping it in the mantle of holistic health.
Iridologists believe that the iris, the colored part of the eye, serves as a sort of dashboard for the body—complete with warning lights, gauges, and dials. They also believe that they can read these warning lights, gauges, and dials to diagnose problems of the entire body. Aside from the fact that this approach simply doesn’t work, many iridologists even disagree with one another as to what part of the iris represents what part of the body. What one iridologist considers to be an indicator of kidney disease may be viewed by another as evidence of a torn kneecap.
For the iridologist, every fleck, spot, color variation, elevation, and depression in the iris is presumed to have diagnostic significance.
Iridologists are fond of diagnosing vague and medieval-sounding diseases such as uterine catarrh, prolapse of the transverse colon, abdominal plethora, lymphatic-rheumatic-tubercular constitution, and all sorts of other mysterious maladies that you will not find listed in any medical dictionary. Most iridologists also just happen to sell the remedies for the diseases they diagnose.
There have been a number of genuine scientific tests of the accuracy, of iridology diagnosis. Iridologists were given an opportunity to diagnose people whose medical histories had already been well documented by more boring, but accurate, medical tests. How did the iridologists do? Well, let’s just say that no one has yet seen a need to add iridology to the curriculum of Harvard Medical School.
An ophthalmoscope lets the doctor look inside the eye to see if there are any diseases or problems such as vitreous floaters or retinal detachments. It was the development of the ophthalmoscope in Germany in the nineteenth century that marked the beginning of modern ophthalmology.
A tonometer tests the intraocular pressure, the pressure produced by fluid inside the eye. High eye pressure is a sign of glaucoma, which can lead to blindness if it isn’t detected early and treated. Some tonometers have a small probe that actually touches the surface of the eye, while others shoot a puff of air into the eye to measure the pressure.
More and more eye doctors—both optometrists and ophthalmologists—are investing in cameras to photograph the inside and outside of the eye. The pictures give the doctor more time to study the eye and look for abnormalities. By letting the doctor document and compare changes from examination to examination, photographs allow more potentially dangerous conditions to be spotted and dealt with earlier.
A tangent screen is used to test peripheral vision—how much you can see happening on your sides while you are looking straight ahead.
With so many ways to precisely measure your vision and screen for possible abnormalities, eye doctors will never have to rely on that old standby—a clear night and a starry sky—again. And there’s no reason for you to remain in the dark about your eyesight any longer, either.