Understanding Your Eyeglass Prescription

Two of the most common abbreviations on any eyeglass prescription are OD and OS. OD stands for oculus dexter, which refers to your right eye. OS stands for oculus sinister, which refers to your left eye. Some prescriptions also include OU, short for oculus uterque, meaning both eyes together. These abbreviations are standard across eye care and appear on prescriptions worldwide.

When looking at your prescription, the OD line is typically listed first, followed by the OS line. Each row contains the sphere, cylinder, and axis values specific to that eye. If you notice that the numbers differ between OD and OS, that simply means each eye needs a slightly different correction, which is perfectly normal.

The OU designation is used when a measurement or instruction applies equally to both eyes. For example, an ADD power for reading may be listed once under OU rather than repeated on separate lines. Pupillary distance measurements may also be noted this way when a single binocular measurement is recorded.

If your prescription includes values in both the cylinder and axis columns, you have astigmatism. The cylinder number tells you how much correction is needed, and the axis tells the lab the exact angle at which to orient that correction. A cylinder value of -0.50 is considered mild, while -3.00 or higher is more significant. Your sphere value is corrected first, and the cylinder and axis are layered on top to produce a fully customized lens.

Pupillary distance is the measurement in millimeters between the centers of your two pupils. It tells the lab where to place the optical center of each lens so it aligns precisely with your line of sight. An incorrect pupillary distance can cause eye strain, headaches, or blurry vision, especially in stronger prescriptions. Your measurement may be listed as a single binocular number or as two monocular values for greater accuracy.

An eyeglass prescription and a contact lens prescription are not interchangeable. Contact lenses sit directly on the surface of your eye, while glasses sit about 12 millimeters away, so the power values often differ. A contact lens prescription also includes a base curve and diameter, which are specific to the lens brand and your eye shape. You will need a separate contact lens fitting and prescription from your optometrist.

In most states, an eyeglass prescription is valid for one to two years from the date of your exam. After that period, you will need a new eye exam to receive an updated prescription. Regular exams are important not only for keeping your correction current but also for monitoring your overall eye health. If you notice vision changes before your prescription expires, schedule an appointment sooner rather than waiting.

A minus sign indicates a correction for nearsightedness, where distant objects appear blurry without glasses. A plus sign indicates a correction for farsightedness, where near objects are harder to focus on. The further the number is from zero in either direction, the stronger the correction.

Common signs include increased squinting, difficulty reading signs at a distance, eye strain during close-up work, or frequent headaches. You may also notice that your current glasses no longer feel as sharp as they once did. We recommend annual comprehensive exams to catch changes early, even if your vision seems stable. An updated prescription can also improve the clarity of prescription sunglasses and specialty coated lenses you may already wear.