Vision is a complex process that involves both the eyes and the brain.  Though eyesight (i.e. the clarity of images formed by the eye) is a big part of how well we see, clear comfortable vision requires additional skills as well.

Eyesight

Eyesight is assessed by visual acuity testing.  At your eye exam, you will be shown charts of letters or numbers across the room and up close.  Distance visual acuity is usually tested at a distance of 20 feet (or a simulated distance of 20 feet using a set of mirrors).  Near visual acuity is typically tested at a distance of 16 inches, which is an average reading distance.

The use of letters of incrementally smaller sizes to evaluate a person's eyesight began in the early 1800's.  The measurement of visual acuity that is used most frequently is the Snellen fraction.  It is named after Dutch Ophthalmologist Hermann Snellen who developed the classic letter chart in 1863.  Since then, people have come to recognize that the Snellen fraction "20/20" means perfect eyesight.

Snellen determined the size of the letters on his test chart by comparing the eyesight of a large number of his patients with that of his assistant, who could see distant objects very clearly.  (Historical accounts don't explain how Snellen knew his assistant had "perfect" eyesight.)

What does "20/20" mean?

The top number in the Snellen fraction is always 20.  It refers to the standardized visual acuity testing distance of 20 feet.  The bottom number of the fraction describes the farthest distance from which a person with normal eyesight can recognize the smallest letters you can see when you are 20 feet from the chart.

Therefore, "20/20" means the smallest letters you can recognize at a distance of 20 feet are the same size as those a person with normal eyesight can see at the same distance.  You have normal (20/20) eyesight.

But if you have 20/40 visual acuity, the smallest letters you can recognize at 20 feet a person with normal eyesight can recognize when they are 40 feet away.  In other words, your visual acuity is only half as clear as that of a person with normal eyesight (20/40 = 50%). 

The larger the bottom number to the Snellen fraction, the worse your eyesight is.  For example, if you have 20/200 visual acuity, the smallest letters you can recognize at 20 feet a person with normal eyesight can recognize when they are 200 feet away.  Your visual acuity is only 10 percent the level of normal eyesight (20/200 = 10%).

It's possible to have eyesight that is better than normal.  In this case, the lower number in the Snellen fraction is less than 20.  For example, if you have 20/15 visual acuity, the smallest letters you can see at 20 feet a person with normal eyesight cannot recognize at 20 feet - they must be closer (15 feet) to recognize them.  Your eyesight is 33% better than normal (20/15 = 133%).

Near visual acuity is also described using a Snellen fraction.  Although the near test distance is typically 16 inches, letters used on the near test card are appropriately sized to give similar relative measurements of eyesight as the distance Snellen fraction.  Therefore, the top number in the Snellen fraction remains 20 for near vision testing, and the fraction is called a "Snellen equivalent."

Sometimes visual acuity is measured using a test distance of 6 meters instead of 20 feet.  In these cases, the Snellen fraction "6/6" indicates normal eyesight (6/6 = 20/20), 6/12 is the same relative level of eyesight as 20/40, and so on.

Binocular Vision

Having 20/20 eyesight doesn't guarantee that you have perfect vision.  Reading an eye chart is a specific, static visual task.  But most daily activities require our eyes to perform more demanding, dynamic functions. 

Clear, comfortable vision depends on how well our eyes work together as a team.  This teaming of the eyes is called binocularity, or binocular vision.  If each eye has 20/20 eyesight but the two eyes are not properly aligned to work together efficiently as a team, blurred vision, eyestrain and headaches can occur.

Computer use and other near vision tasks require our eyes to work together while pointing slightly inward.  This inward direction of the line of sight of each eye is called convergence.

Reading and computer work also require our eyes to move quickly and accurately from point to point on a page or screen.  And the closer objects are, the more focusing effort our eyes have to perform.

Many visual tasks also require that we change the focus and position of our eyes quickly and repeatedly (taking notes during a classroom lecture, for example).  This changing of the focus point of our eyes is called Accommodation.

People who have perfect 20/20 eyesight can have binocular vision problems that can cause headaches, eyestrain, and blurred vision.  Binocular vision problems may also cause reading problems among school-aged children. 

If you or one of your children are experiencing frequent headaches and eyestrain, see your eye doctor to determine if you have a binocular vision or accommodative problem.  Many binocularity and focusing problems can be successfully treated with vision therapy or special prescription eyewear.