Eyeglasses are prescribed by a doctor of optometry or ophthalmology. After an eye exam, your eye doctor will write out and give you a prescription that contains numbers that show the refractive power needed to make a pair of glasses or contacts for your specific vision correction.
Abbreviations
Most optical prescriptions contain the letters OD and OS. OD is the Latin medical abbreviation for right eye, and OS is the abbreviation for left eye. Your prescription may also contain the letters DV and NV, which simply mean distance vision and near vision.
Eyeglass prescriptions almost always have a measurement called a PD, which is the abbreviation for pupillary distance. Sometimes the doctor will not record the PD and will let the optician that fits you for glasses do the measurement. The PD specifies where to put the optical center of the lens in your glasses.
Sphere Power
Optical prescriptions are written as a unit of measurement called a diopter. The higher the number, the stronger the lens. The diopter is written under the category of SPH, or spherical power, for each eye. This number may be anywhere from 0.00 to 20.00 or higher and is measured in .25 steps. The number may be written as a negative for nearsightedness, also called myopia, or as a positive for farsightedness, or hyperopia. If you have no need for correction in the distance in either eye, your doctor may write the prescription as PLANO or PL instead of 0.00, according to usaeyes.com.
Cylinder Power
Eyeglass prescriptions always contain spherical power and may also have other numbers written under cylinder and axis. If there are numbers recorded for cylinder and axis, this means you have an irregularly shaped cornea, which is called astigmatism. Astigmatic correction is very common, and the numbers on the prescription let the optician and optical lab know how much cylinder to put into the lens to help light focus correctly on the retina. Cylinder is also recorded in diopters and can be plus or minus. The axis is where the cylinder is located, anywhere from 0 to 180 degrees.
ADD Power
If your prescription has an ADD power, then you need lenses for reading and close work. The ADD is the abbreviation for addition and is always written in a positive or plus diopter number. This may also be written as NV or near vision correction. The optical lab will add the power of the ADD to your distance correction as a bifocal. It is possible to have no prescription in the distance and just have an ADD or NV prescription. This means you only need glasses for reading.
Prism and Base
Prism and base are not always used in a prescription. According to opticsplanet.net, prism is used to correct alignment problems in the eyes. If one or both of your eyes do not look straight ahead, prism can be added to the lens to force or pull the eye into looking forward. Base is the placement of the prism, either BI (base in) or BO (base out).