Meredith McClellan, age 15, of Patterson, N.J., for her question:
WHAT IS 20/20 VISION?
A test for visual acuity, or sharpness of vision, is usually given with a standard eye chart that is made up of rows of letters. The letters in each line are smaller than those in the line above. If a person being tested has normal vision, he or she is given a rating that is called "twenty twenty" that is written like this: 20/20.
The eye chart is placed 20 feet from the patient being tested. The patient then reads each line first with both eyes and then with the right eye covered. A final test is to read as far down the chart as possible with the left eye covered.
If the person being tested can read letters of the size normal for a person 20 feet away, his vision is recorded as 20/20.
If the person, standing at 20 feet, can read only the letters that the normal eye can see at 40 feet, his vision is said to be 20/40.
Some people have sharper eyesight than normal. A person who can see letters from 20 feet that the normal eye can see only from 15 feet, has 20/15 vision.
In countries where the metric system is used, the chart is read from a distance of 6 meters. Under this system, normal vision is expressed as 5/6.
Astigmatism, which results in blurred vision, and color blindness, or the inability to see certain colors, are other defects in vision that may be tested with charts of various kinds.
The inside of the eye is examined by doctors using instruments called ophthalmoscopes. An ophthalmoscope shows conditions within the eye as clearly as if they were on the surface. It allows the doctor to examine the blood vessels in the eye. The condition of these vessels reveals certain general conditions of the body's circulatory system.
Vision is the ability of the eye to form an image of an object and the ability to then send the image to the visual centers of the brain.
The eye works very much like a camera. The eye and the camera each obtain pictures in similar ways. Each has a lens to focus the image.
The iris of the eye and the diaphragm of the camera each regulate the amount of light received. The retina of the eye and the film of the camera are each sensitive to light.
As a person grows older, the lens of the eye is less elastic and it loses some of its ability to change the focus. This condition is called presbyopia. A lot of people become prebyophic when they near the age of 40 and they then need glasses for reading.
Doctors tell us that good habits are important in using our eyes. To read comfortably, good light is very necessary. We should then hold the printed page at least 14 inches from our eyes.
In reading, the eyes should also be rested from time to time since the muscles that focus the eyes can often become tired after long use. When this happens, they do not work properly. Doctors stress the importance of guarding against eyestrain.