Eyeglasses are more than just a fashion statement; they're a necessity for many. But when it comes to understanding your eyeglass prescription, things can get a bit tricky. This comprehensive guide aims to demystify the jargon and numbers, helping you make sense of your prescription. We'll cover everything from the basic components of a prescription to how to interpret the various measurements.
The Basics of an Eyeglass Prescription
An eyeglass prescription is a document provided by an optometrist or ophthalmologist. It contains specific information about your vision and the corrective lenses you need. The prescription typically includes several abbreviations and numbers, each representing a different aspect of your vision.
The first term you'll likely encounter is "OD," which stands for "oculus dexter." This Latin term refers to your right eye. Similarly, "OS" stands for "oculus sinister," which refers to your left eye. Occasionally, you might see "OU," which stands for "oculus uterque," or both eyes.
Next, you'll see a series of numbers. These represent the lens power needed to correct your vision. The unit of measurement is diopters (D). A positive number indicates hyperopia (farsightedness), while a negative number suggests myopia (nearsightedness).
The term "cylinder" (CYL) refers to the lens power for astigmatism. If this section is blank, you likely don't have astigmatism. The "axis" is a number between 1 and 180, indicating the orientation of the astigmatism.
Lastly, "add" is the additional magnifying power for the lower part of multifocal lenses. It's typically a number between +0.75 to +3.00 D and is the same for both eyes.
Understanding the Numbers
Now that we've covered the basic terms, let's delve deeper into the numbers on your prescription. The first number you'll see is for sphere (SPH). This indicates the degree of shortsightedness or longsightedness. A minus sign (-) before the number means you're shortsighted, and a plus sign (+) means you're longsighted.
The cylinder (CYL) number represents the degree of astigmatism. Like the sphere number, a minus sign indicates shortsighted astigmatism, and a plus sign indicates longsighted astigmatism.
The axis number, which accompanies the cylinder number, represents the orientation of the astigmatism. It's measured in degrees from 1 to 180. The number 90 corresponds to the vertical meridian of the eye, and the number 180 corresponds to the horizontal meridian.
The "add" number is used for bifocal or progressive lenses. It's the additional corrective power that helps with close-up work, like reading. This number is always a plus.
Prism and Base
Some prescriptions may include a "prism" and "base." The prism number, measured in prism diopters (p.d. or Δ), is used to correct eye alignment problems. The base indicates the direction of the prism. It can be up (BU), down (BD), in (BI), or out (BO).