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Decoding Your Eyeglass Prescription

When you go to the doctor for an illness, it’s pretty easy to read the prescription he writes for you. Even if your doctor is notorious for scribbled handwriting, more often than not you can read what medication he or she has prescribed. But have you ever tried reading an eyeglass prescription from your optometrist or ophthalmologist? Not so easy. With so many letters and numbers (and not to mention plus signs and minus signs), it can be confusing. That’s why we put together the following guide to help you dissect your eyeglasses prescription.

OD and OS:
First, notice the letters “OD” and “OS” on your eyeglasses prescription.  These are Latin abbreviations for the terms oculus dextrus, meaning right eye, and oculus sinister, meaning left eye. Sometimes a prescription will include the letters “OU,” oculus unitas meaning both eyes. While using the Latin abbreviations is traditional for eyeglass prescriptions, some doctors have started modernizing their prescriptions by using RE (right eye) and LE (left eye).

Sphere:
These numbers are measured in diopters (D) and represent the amount of lens power needed to correct your vision. Typically, the farther away the number is from zero, the more vision correction you need. If there is a plus sign (+) in front of the number, it means you are farsighted and have more trouble seeing things close up. If there is a minus sign (-) in front of the number, it means you are nearsighted and have more trouble seeing at a distance.

Cylinder:
This number refers to the amount of astigmatism in your eyes. Astigmatism is a condition where the eye does not focus light evenly onto the retina, causing images to blur or stretch. The cylinder number is most commonly preceded by a minus sign and represents the difference in power that exists as you measure how the eye focuses light. If this column is blank, then you have no astigmatism or it is too slight to need correction.

Axis:
The Axis number describes the position of the astigmatism in your eye.  This number shows the direction the eye doctor needs to position any cylindrical power in your lenses (required for astigmatism only). Axis numbers are measured in angle degrees ranging from 1 to 180.  If the Cylinder section on your prescription is blank, then you will not have an Axis number as the prescription is spherical.  The idea is to neutralize the difference in power in the eye with the power in the lens.

Prism:
The prism numbers represent the amount of prismatic power needed for your eyes. Prismatic powers are prescribed to help with eye alignment problems (vertical and horizontal). The need for correcting eye alignment is uncommon, so only a small amount of eyeglass prescriptions include prism.

Add:
These numbers refer to additional (magnifying) power needed to correct vision up close presbyopia. The magnifying power increases as your eye moves down the corridor of the lens in the case of multifocal lenses. The add number is always a plus power, even if it is not preceded by a plus sign on your prescription. It will also most often be the same number for both eyes.

PD:
The pupillary distance, or PD, is the distance between the centers of each pupil (black hole). Once you reach adulthood, this number does not change. The PD number is very important for properly fitting lenses and is required for optimum visual comfort and performance.  The measurement of the proper PD enables the center of the lens to align with the center of the pupil.