Visual acuity is usually measured with a Snellen chart. Snellen charts display letters of progressively smaller size. "Normal" vision is 20/20. This means that the test subject sees the same line of letters at 20 feet that person with normal vision sees at 20 feet. 20/40 vision means that the test subject sees at 20 feet what a person with normal vision sees at 40 feet. Another way of saying this is that a person with 20/40 vision has vision that is only half as good as normal - or, objects must be at half the normal distance for him to see them. A person with 20/20 vision is able to see letters 1/10th as large as someone with 20/200 vision. 20/20 is not the best possible eyesight however, for example, 20/15 vision is better than 20/20. A person with 20/15 vision can see objects at 20 feet that a person with 20/20 vision can only see at 15 feet.
20/20 - Normal vision. Fighter pilot minimum. Required to read the stock quotes in the newspaper, or numbers in the telephone book.
The refractive error of the eye can be expressed in numeric terms. The power of the lenses necessary to correct your vision is measured in units called diopters (see below). The first number designates the amount of myopia (minus numbers) or hyperopia (plus numbers). The second number (if present), indicates the amount of astigmatism. The third number shows the axis of the astigmatism. Depending on the sign of the second number, the axis indicates the orientation of the steepest or flattest meridian of the cornea. The fourth number is the "add," or the amount of magnification needed to give clear vision at near. If this number is present, bifocals have been prescribed to correct presbyopia.
A diopter is a unit of measurement of the refractive error. It may be a negative number (myopia, nearsightedness), or a positive number (hyperopia, farsightedness). A -1.00 diopter myope is able to see objects at 1 meter clearly. A -2.00 diopter myope is able to see objects at a ½ meter clearly. The greater the myopia, the greater the strength of the lens in diopters needed to correct the refractive error, and the closer an object must be to be viewed clearly.