That familiar chart topped by the large letter "E" is an important measure of your visual acuity, or sharpness of vision. If your doctor says your visual acuity is 20/20, you see at 20 feet what a person without vision problems sees at 20 feet. This measurement is traditionally considered the baseline, and any differential from it is used by your doctor to evaluate your vision. Accordingly, if you have 20/40 vision, you see at 20 feet what a person with 20/20 vision would see at 40 feet. And if you're one of the lucky few (someone with 20/15 vision, for example), you can see at 20 feet what others would have to be at 15 feet to see. This test shows how well you can distinguish the shape of an object.
Beyond 20/20 Vision: Interpreting Your Prescription
When doctors use the phrase “20/20 vision,” they are measuring visual acuity, but doctors use a different notation when taking a measurement of the prescription of the eye. Doctors compare how the shape of your eye differs from one with perfect vision. This difference is assigned a number that expresses the severity of the error. The number is named a diopter. An eye that does not require corrective lenses is assigned the number (0), and the measurable range of correction is from (+14) to (-14), depending on the aberrations your eye has.
Your vision prescription consists of three numbers. Each eye is given its own set of numbers. Let's use a typical prescription as an example:
-5.00 -1.50 x 180
The first number (-5.00) indicates the shape or length of your eye. A number with a minus sign (-) before it means you are nearsighted, or short sighted. A number with a plus sign (+) means you are farsighted, or long sighted. The farther the number is away from zero, the more severe the nearsightedness or farsightedness.
The second number (-1.50) reflects the irregularities in the eye, usually associated with the cornea, and represents the amount of astigmatism that you have. A number with a minus sign (-) or a plus sign (+) may be used depending on the convention your doctor uses.
The third number (180) describes the axis or direction of your astigmatism.
So in this example, you do not have 20/20 vision, you are moderately nearsighted, with a moderate degree of astigmatism.
