How to Read a Prescription

What do all these letters and numbers mean?

OD: OD is short for the Latin term oculus dexter which means right eye.

OS: OS is short for the Latin term oculus sinister which means left eye.

Sphere: The sphere (SPH) on your prescription indicates the lens power you need to see clearly. A minus (-) symbol next to this number means you’re nearsighted, and a plus (+) symbol means the prescription is meant to correct farsightedness.

Cylinder: The cylinder (CYL) number indicates the lens power needed to correct astigmatism. If this column is blank, it means you don’t have an astigmatism.

Axis: An axis number will also be included if you have an astigmatism. This number shows the angle of the lens that shouldn’t feature a cylinder power to correct your astigmatism.

Prism: A prism number indicates the amount of prismatic power, measured in diopters, needed to compensate for any alignment problems in your eyes.

Base: This will either appear in a column of its own or be included in the prism column. This column indicates the direction of a prism by noting the relative position of its thickest edge. The orientation of the prism will be indicated by ‘BU’ (base up), ‘BD’ (base down), ‘BI’ (base in), or ‘BO’ (base out).

Add: The number associated with either of these terms indicates the additive power needed to correct your immediate near vision if your prescription is primarily for distance correction.

Pupilary distance (PD): PD, or pupillary distance, refers to the distance in millimeters between the center of one pupil to the center of the other. Having a correct PD on your glasses prescription ensures that you are looking through the ideal spot in your lenses. If this number was not provided on your prescription by your eye care professional, you can measure it yourself.

How to measure PD yourself

1. Print our downloadable PD ruler

2. Place the ruler directly over the center of your right pupil so that the ruler is horizontal. Place against forehead for added stability. Stand in front of a mirror, or ask someone else to measure for you.

3. While looking straight ahead, measure the distance from the center of your right pupil to the center of your left pupil.

4. Repeat a couple of times for accuracy and use an average for your final measurement.

Nearsighted prescriptions

A nearsighted prescription will feature a number with a minus (-) symbol in the ‘sphere’ box. This means your lenses will be shaped to improve your distance vision.

Farsighted prescriptions

A prescription for someone who is farsighted will feature a number with a plus (+) symbol in the ‘sphere’ box. This indicates that your lens should be shaped to correct near vision.

Astigmatism prescriptions

If you have an astigmatism, you will have a number in the ‘cylinder’ column of your prescription that indicates the lens power needed to correct the astigmatism. There will also be a number in the ‘axis’ column that’s needed for astigmatism correction.

Bifocal & progressive prescriptions (multifocal)

A multifocal prescription will include an ADD value, sometimes marked as NV. This indicates the required strength for the near prescription in your lens.

Prescriptions with prism correction

A prescription for correcting eye alignment issues will include a PRISM and BASE value. These are needed to tell us how to shape the lens so that it can accurately align the wearer’s eyesight. The base value will show either ‘in’ or ‘out’, or ‘up’ and ‘down’, indicating the direction the prism should be angled.