Reading your own prescription: what sphere, cylinder, axis and PD actually mean

A prescription slip full of abbreviations and decimal numbers is handed to most patients at the end of an eye test with little explanation of what any of it means. Here's a plain-language walkthrough of the numbers, so you can read your own results the next time you get one.

OD and OS: which eye is which

OD stands for oculus dexter, the right eye, and OS stands for oculus sinister, the left eye. Some newer prescriptions use "RE" and "LE" instead, which mean the same thing in plain English. It's a small detail, but reading the wrong row for the wrong eye is a real and easy mistake to make.

Sphere: the main correction number

The sphere value, usually labeled SPH, is the core of your prescription: how strong a correction each eye needs for basic clarity. A minus number means nearsightedness, so distant objects are blurry; a plus number means farsightedness, so close-up objects need the most help. The size of the number, not the sign, indicates how strong the correction is; a small number close to zero is a mild prescription, and a larger number, positive or negative, means a stronger one.

Cylinder and axis: correcting astigmatism

If your prescription has a value under CYL, cylinder, that means the front of your eye isn't perfectly round, it's shaped more like a rugby ball than a basketball in one direction, and that's astigmatism. The cylinder number states how much correction that irregular shape needs. The AXIS number, given in degrees from 0 to 180, tells the lens exactly which direction across the eye that correction needs to sit. Get the axis wrong by even a few degrees and a lens can feel off in a way that's hard to describe, which is why this pair of numbers, cylinder and axis, has to be measured precisely rather than estimated.

ADD: the reading power for presbyopia

If you're over 40 or wear bifocals or progressives, you'll likely see an ADD value. This is extra magnifying power added on top of your main prescription, specifically for reading and close-up work, and it's what a progressive or bifocal lens builds into its lower zone.

PD: the measurement that decides how the lens sits

Pupillary distance, PD, is the measurement in millimeters between the centers of your two pupils. It isn't a correction value at all, it's a fitting measurement, and it's what tells the lab exactly where to position the optical center of each lens inside your frame. Get this measurement wrong and even a perfectly accurate prescription can feel slightly off, since you'd be looking through a spot on the lens that isn't quite centered over your eye.

Why this is worth knowing before your next visit

None of this is about second-guessing your optometrist, it's about being able to follow along. Knowing that your cylinder went up, or your ADD increased since last year, gives you a concrete way to ask what changed and why, rather than nodding along at numbers that mean nothing on the page. It also means you can compare your own results year to year, which is a real, checkable way to track your own eye health over time.

Book the exam, then ask by name

A Vision7 comprehensive eye test is where these numbers come from in the first place, specifically the subjective and objective refraction steps, and it's worth asking your optometrist to walk you through your own results rather than just handing you the slip. If your results show astigmatism, ask about Shamir's Opulens Exact at the counter, made to your exact cylinder and axis measurement rather than a standard curve. Every pair includes a 1-year warranty and free lifetime custom fitting, useful once your new PD and prescription are set into a frame.