What is 20/20 Vision?
Most of us are familiar with the term "20/20 vision" and know it means our eyesight is pretty good.
But what exactly does "20/20" mean? Is it possible to have vision that's better than 20/20? Is it safe to drive if you don't have 20/20 vision? And what does it mean if your vision is 20/40? Or 20/200?
Also, can you have 20/20 eyesight and still have vision problems? (Spoiler alert: The answer is "yes.")
It All Started With an Eye Chart
In 1862, a Dutch eye doctor named Hermann Snellen devised an eye chart that became the standardized way eye care professionals to this day determine a person's distance visual acuity--how clearly someone sees distant objects.
The traditional Snellen eye chart has a single large block letter (usually an "E") on the top of the chart, and 10 subsequent lines of progressively smaller block letters. Each line has more letters than the previous line to keep spacing between the letters relatively uniform.
By experimentation, Snellen determined the smallest letters a person with "normal" vision could consistently identify correctly. This became the 20/20 line on the chart. In other words, if you can correctly identify letters of this size on the eye chart (but none smaller) from a testing distance of 20 feet, you have normal (or "20/20") vision.
Since the standard testing distance for the Snellen chart used in the United States is 20 feet, the top number in the Snellen fraction is always "20." The bottom number varies, depending on the size of the letters on each row of the chart.
Here's how it works
Three lines above the 20/20 line on the Snellen chart, the letters are twice as large as those on the 20/20 line. A person with normal visual acuity therefore can be expected to correctly identify these larger letters from a viewing distance that is roughly twice that of the normal testing distance of 20 feet (so, 40 feet).
If this line is the smallest line of letters a person can correctly identify at the normal testing distance, he or she has "20/40" vision--this is the smallest line of letters they can see at 20 feet, but a person with normal vision can correctly identify these letters at a distance of 40 feet.
So the larger letters on the Snellen chart are designated with visual acuity fractions that have a larger second (bottom) number, which is the maximum viewing distance from which a person with normal vision can correctly identify the letters on that line of the chart.
The "big E" on the top line of the traditional Snellen chart is the 20/200 line. If this is the smallest line on the chart you can correctly identify from a testing distance of 20 feet, your distance vision is very blurred. How blurry? A person with normal vision could stand 200 feet from the chart and still be able to identify the letter. Roughly speaking, that person with normal vision has distance visual acuity that is 10 times clearer than yours.
Is it possible to have better-than-normal visual acuity? Yes!
The traditional Snellen eye chart has three lines of letters below the 20/20 line that contain letters that are smaller than those on the 20/20 line. The letters on the bottom line are half the size of "20/20" letters.
If you are fortunate enough to be able to identify the letters on this smallest (20/10) line, it means your distance visual acuity is twice as good as that of a person with "normal" vision. In other words, you can identify these very small letters from the standard testing distance of 20 feet, but a person with normal vision would have to be twice as close to the chart--only 10 feet away--to be able to correctly identify them.
How Visual Acuity Is Measured
Visual acuity usually is measured one eye at a time. The person having their vision tested covers one eye and reads aloud the letters on each row, beginning at the top. The smallest row of letters he or she can correctly identify indicates the distance visual acuity of that eye.
The traditional Snellen chart used in the United States has 11 rows of letters, with the following visual acuity designations for each row:
A common variation of this chart is the "tumbling E" chart, which contains multiple lines containing the block letter "E," in progressively smaller sizes and displayed in varying spatial orientations (rotated in increments of 90 degrees). This chart often is used to test the visual acuity of children who are too young to fully know all the letters of the alphabet or to test the eyesight of illiterate adults.
Instead of saying the name of letters aloud, the child or adult being tested indicates with their extended fingers the direction of the "arms" of each "E" on the chart (up, down, right or left). Studies have shown visual acuity results obtained from a tumbling E chart correlate well with those obtained from a standard Snellen eye chart.
In Europe, Snellen visual acuity testing usually is recorded with metric notations, using a standard testing distance of 6 meters. In this system, a visual acuity measurement of 6/6 is equivalent to an American measurement of 20/20, 6/12 is equivalent to 20/40, and so on.
And in case you're wondering if your eye doctor can correctly measure your distance visual acuity in an exam room smaller than 20 feet in length, don't worry. With the help of mirrors and a projector, your eye care professional can simulate a standard testing distance of 20 feet in a room half that size--or even smaller!
By the way, to be granted an unrestricted driver's license in most of the United States, your best-corrected visual acuity--that is, your distance visual acuity measurements obtained while you are wearing your eyeglasses or contact lenses (if you need vision correction)--must be 20/40 or better.
Good Vision Is More Than Visual Acuity
Distance visual acuity is only one aspect of good vision. Having 20/20 eyesight does not mean you have perfect vision.
Other important components of clear, comfortable vision include the ability to change focus and maintain focus on near objects, depth perception, peripheral vision, accurate eye movement and alignment, and more.
To see the world as clearly as possible and enjoy a lifetime of good vision, be sure to see your eye doctor for regularly scheduled comprehensive eye exams.
Article ©2013, Access Media Group LLC. Source: The Eye Chart and 20/20 Vision by AllAboutVision.com.
VISION & HEALTH NEWSLETTER COURTESY OF:
Stephen Cohen, O.D.
10900 N Scottsdale Road