According to Refractive Error Source, it is common to have some degree of imperfect vision because light does not focus correctly on most people's retinas. Most vision issues can be corrected with eyeglasses. Every eyeglass prescription has a set of numbers and abbreviations that are used to identify what is needed to correct a person's vision. The numbers tell whether the prescription is made for nearsighted, farsighted, astigmatism, bifocal or other vision corrections, also called refractive errors.
Eyeglass prescriptions are written in a standardized way to prevent confusion and misinterpretation. The prescription for the right eye--written as OD, which is Latin for Oculus Dexte--is always noted first. OS, or Oculus Sinister, is the left eye. Occasionally a doctor will use the abbreviation RE for Right Eye and LE for left eye, according to The Ohio State University Medical Center. The prescription for each eye includes numbers indicating sphere, cylinder and axis. It sometimes includes other information such as bifocal power or prism correction.
An optical prescription notes whether a person is nearsighted--myopic--or farsighted--hyperopic. This number appears under "Sphere" or "SPH" on a prescription and indicates the power of the spectacle lenses. The power refers to diopter, which is a measure of the refractive strength of a lens or of an eye. The Opticianworks website states that sphere powers in ordinary prescriptions are written with the numbers from +/- 0.25 diopters to approximately +/- 15.00 diopters. Prescriptions above the range of +/- 15.00 diopters may require special lenses.
Nearsightedness is designated by a negative number, for example -1.00, and farsightedness is written as a positive number. The greater the number, the stronger the prescription and power required to see.
When the clear outer layer of the eye is irregularly shaped, the condition is called astigmatism. A perfect cornea is baseball shaped but most corneas tend to be more football-shaped according to the National Eye Institute.
Cylinder numbers in a prescription represent the amount of astigmatism. Cylinder powers in ordinary prescriptions range from +/- 0.25 diopters to approximately +/- 5.00 diopters. Most optometrists write cylinder in the negative form, such as -1.00, and most ophthalmologists write cylinder as a positive number, like +1.00.
The cylinder number also corresponds with the Axis number on a prescription and will be a number between 1 and 180, depending on the curve of the cornea.
It is possible to have astigmatism without nearsighted or farsighted correction. In this case, a person's prescription would have 0.00 written under Sphere and have numbers under cylinder and axis.
Eyeglass prescriptions contain a section after sphere, cylinder and axis called "ADD" which stands for "Addition" or additional power. The numbers in this field indicate the bifocal power of a prescription and are always written as a positive number.
Bifocals usually correct for presbyopia, a condition that begins around the age of 40 to 45, according to optometrist Dr. Ted Montgomery. Loss of elasticity of the crystalline lens causes a reduction in the inability to focus clearly up close. This is different than being farsighted and eventually happens to everyone, even if they are nearsighted or farsighted. The higher the ADD number, the more power required to read.
Some eyeglass prescriptions include numbers written under the column Prism. This part of the prescription represents a correction for a muscle imbalance in the eye. The Allaboutvision website states that prism is indicated in either metric or fractional English units such as 0.5 or ½, and the direction of the base of prism is indicated by noting it as Base Up or Down (BU or BD) or Base In or Out (BI or BO).