How To Read Your Prescription

“What Do All Those Numbers Mean, Anyway?”

By John Dick ABO, FNAO

Prescriptions we receive from our eyecare professionals are not generally something we take time to consider. We leave it up to the professionals. But if you take time to look at your prescription, it will tell you a lot about your eyes. Let’s take a look at the following examples:

Spectacle Prescription

Sphere Cylinder Axis Prism
OD -2.00 -1.00 013
OS -2.50 -1.25 021
Add -2.50

PD: 68/65

Contact Lens Prescription

Contact Lens Diameter Radius Power
OD Ciba Air Optix Aqua 8.6 14.2 +2.00
OS Ciba Air Optix Aqua 8.6 14.2 +1.75

Prescription Terminology

Let’s begin breaking down the prescriptions above by looking at the information in bold print:

OD = ocular dexter = right eye
OS = ocular sinister = left eye
Sphere = degree of near-sightedness/far-sightedness throughout the horizontal plane of the lens
Cylinder = degree of astigmatism correction in the vertical plane of the lens
Axis = orientation (in degrees) of where the cylinder (or astigmatism) correction resides in the lens
Add = amount of power “added” to the prescription to allow presbyopic patients to see up close

Differences Between Contact Lens Prescriptions and Spectacle Prescriptions

Contact lens prescriptions are different in they denote a particular brand of contacts and include the power of the lenses as well as the diameter and radius of the lenses. Contacts may also be prescribed for patients with astigmatism or presbyopia as with the spectacle prescription. The diameter and radius of a contact are of vital importance in making sure the lenses stay centered and fit the curvature of your corneas. Contact prescriptions may differ from spectacle prescriptions due to vertex distance (the distance between the surface of the eye and the lens). Obviously, contacts will rest on the surface of the cornea whereas spectacles lenses will generally be about 12 mm from the surface of the cornea to the back of the lens. This accounts for the differences in power between the two lenses.

Prescription Facts

It’s important to note a few things about prescriptions:

1) prescriptions are governed by the Food and Drug Administration and must be signed, dated and have an expiration date. Since contacts lens complications may arise quickly. They are only good for one year. Generally, spectacle prescriptions are good for two years unless medical necessity dictates an annual exam.

2) Prescriptions are written in a standard format as exampled above. This prevents confusion maintains uniformity across professional lines.

3) Tolerances have been established by the American National Standards Institute to ensure patients receive the lens quality and prescriptions necessary to correct their vision.