Since your first eye exam, you've heard fractions like 20/20 or 20/40 to describe your vision. But what do they really mean?

Defining 20/20 vision

Simply, 20/20 means that you can see at 20 feet what you should be able to see at 20 feet, says A. Tim Johnson, MD, associate professor of ophthalmology and director of Comprehensive Ophthalmology Services at University of Iowa Hospitals & Clinics.

The term, 20/20, is an expression of your ability to see at a distance. The first number shows your distance from an eye chart. The second number shows the size letter you can read.

For example, 20/30 indicates vision problems. Though you're 20 feet from the chart, you read letters which most people see when they're farther--30 feet--from the chart. On the other hand, 20/15 shows sharper than average vision.

Why 20 feet? When you're looking at an object 20 feet away, your eye is relaxed in its normal shape, Dr. Johnson says. It does not have to bend the light rays from the object to focus the image on the retina in the back of the eye. To see objects closer than 20 feet, the lens must bend the light rays.

How common is 20/20 vision?

Only about 35 percent of all adults have 20/20 vision without glasses, contact lenses or corrective surgery. With corrective measures, approximately 75 percent of adults have this degree of visual acuity while the other 25 percent of the population just doesn't see very well, Dr. Johnson says.

Visual acuity develops early in life, about six months of age. Some children lose their 20/20 vision about age eight or nine when they become nearsighted. Generally, visual acuity then remains stable throughout your life, with perhaps only a slight decrease in the sixth or seventh decade, explains the ophthalmologist.

Losing your near vision with age

Though you won't lose your 20/20 vision as you approach middle age, you will most likely lose your near vision. The eye's lens becomes less flexible and focusing on close objects is more difficult. Reading glasses or bifocals usually help.