Decoding Your Eyeglass Prescription: What Exactly Are You Seeing?

“Is the image blacker and sharper on number one … or number two; one … or two; one … or two?” Fifteen minutes of grilling have passed, and you have walked out of your eye doctor’s office with a prescription for eyeglasses. It is certainly more legible, in most cases, than whatever it is that a physician scribbles on their RX pad, but what exactly are you seeing when you read an eyeglass prescription?

What are OD and OS on an eyeglass prescription?

These two abbreviations are Latin terms relating to your right eye and left eye. Your right eye is oculus dexter (OD) and your left eye is oculus sinister (OS). Each eye is measured independently utilizing various criteria and deviations from an established norm and its deviation is recorded under each of the individual columns in order to help establish how the corrective lenses will be constructed.

What do those geometric shapes mean?

The columns on your eyeglass prescription chart are labeled with geometric shapes. Each one is a part of the specified instructions for creating your corrective lenses according to how you reacted to the series of lenses tested on each eye. The grilling you went through had a purpose. Here’s what they mean:

• Sphere (SPH): This indicates the number of diopters (measurement of lens power) needed to correct nearsightedness or farsightedness. If the number is negative (-), you are nearsighted and if it is positive (+), you are farsighted.
• Cylinder (CYL): The number in this column, if there is one, is the number of diopters necessary to correct astigmatism. This space will be blank or crossed out if there is no astigmatism to be corrected.
• Axis: If your prescription has numbers for CYL, then it will also have a number in the axis column. This number describes the point on the lens meridian where there is no power to correct astigmatism.
• Prism: Eye alignment problems are compensated for by adding prismatic power to the prescription.
• Add: This column is for magnification power, which is added to the bottom portion of the lens in multifocal lenses. The number is similar to what you find on a pair of reading glasses.

What is PD?

Pupillary distance (PD) is a measurement of distance between the centerline of your pupils. Represented in millimeters, this number is critical for establishing the center point of each lens in relationship to the center point of your eyes. Without this measurement, your lenses would not be focused properly to correct your vision.

Now you know what you’re seeing whenever you walk out of your eye doctor’s office with a prescription in your hand. Contact Michigan Eye Institute with any additional questions concerning your eyeglass prescription as well as any other vision-related questions you might have.