Glasses and Eye Terms Glossary: PD, OD, OS, SPH, and More

The first time you order glasses online, the form reads like it's in another language. Latin abbreviations, plus and minus numbers, little symbols you've never seen on anything else. This page translates the lot into plain English, grouped by what each term actually relates to, so you can read a prescription and fill out an order without second-guessing yourself.

If you'd rather see a prescription explained as a whole, we have a separate walkthrough on how to read your eyeglass prescription. And for everything about the PD measurement specifically, there's our guide to measuring pupillary distance.

Which eye is which

OD (oculus dexter) — Latin for your right eye. Anything in the OD row is for the right lens.

OS (oculus sinister) — Latin for your left eye.

OU (oculus uterque) — Both eyes together. You'll see this less often.

The numbers that set lens power

SPH (Sphere) — The main event: the lens power, in diopters, that fixes nearsightedness or farsightedness. A minus value is nearsighted, a plus value is farsighted. If you see "Plano," "PL," or "DS" here, it just means that eye needs no spherical correction — not that something's missing.

CYL (Cylinder) — The power that corrects astigmatism. A blank here simply means you don't have it.

Axis — Comes along with cylinder, never without. It's a number from 1 to 180 marking the angle the astigmatism correction sits at.

ADD (Addition) — Extra close-up power for reading, found on progressive, bifocal, and reading prescriptions. Always positive.

Prism / Base — An uncommon correction for eye-alignment trouble, shifting the image so the eyes team up properly. Noted with a direction, like "Base Out."

Diopter — The unit all this power is measured in. Bigger number, stronger lens.

How the lens lines up with your eyes

PD (Pupillary Distance) — The distance in millimeters between the centers of your pupils. It's what tells the lab where to put the optical center of each lens. Notably, it's often not on your prescription — more on why below.

Binocular (single) PD — One number for both pupils together, like 63.

Monocular (dual) PD — Two numbers, each pupil to the bridge of the nose, like 31.5 / 31.5. More precise, and the better choice for progressives. We compare the two in single PD vs dual PD.

Near PD — A slightly smaller PD for reading glasses, since your eyes turn inward for close work — usually about 3 mm less than your distance PD. More in near PD for reading glasses.

Optical center — The sweet spot of the lens, the point built to give you the clearest view. Your PD is how it gets lined up with your pupil.

Vertex distance — The little gap between the back of the lens and your eye. Matters mainly for strong prescriptions.

Types of lenses

Single-vision — One power across the whole lens, for one viewing distance. The most common type.

Bifocal — Two zones with a visible line: distance up top, reading below.

Progressive — Distance, intermediate, and near all blended together, no line. Takes some getting used to, but many people prefer the seamless look.

Reading glasses — Single-vision lenses tuned for close-up, built around your near PD.

Lens index — A number like 1.5, 1.61, 1.67, or 1.74 describing how thin the material is for a given power. Higher index, thinner lens — handy for strong prescriptions that would otherwise be bulky.

The frame itself

Bridge — The bit that rests on your nose.

Temples (arms) — The two pieces that reach back over your ears.

Frame measurements — Those tiny numbers stamped inside the arm, like 52□18-140 (lens width, bridge width, temple length). Worth knowing: these describe the frame, not your eyes. They are not your PD, even though people mix the two up constantly.

Why your PD often isn't on the prescription

One thing trips people up more than any other term here, so it's worth spelling out. You'd reasonably expect your PD to be printed on your prescription — but frequently it isn't, and that's not an oversight. PD isn't part of the refraction (the part of the exam that produces your lens powers); it's considered a fitting measurement taken when dispensing glasses. In the United States, in fact, only a handful of states require it on the prescription at all. So if yours is missing, you're in good company — you can ask the clinic for it, dig it out of a past order, or measure it yourself.

In short

Strip away the jargon and it's really just five questions: which eye (OD/OS), how strong (SPH, CYL, axis, ADD), how it aligns to you (PD, optical center), what kind of lens (single-vision, bifocal, progressive), and how big the frame is. Get those straight and the order form stops being intimidating.

Sources

• U.S. Federal Trade Commission, Complying with the Eyeglass Rule — confirms PD is a dispensing measurement, not part of the refraction, and that only some states require it on a prescription: https://www.ftc.gov/business-guidance/resources/complying-eyeglass-rule
• FTC Consumer Advice, Buying Prescription Glasses or Contact Lenses: Your Rights: https://consumer.ftc.gov/articles/buying-prescription-glasses-or-contact-lenses-your-rights

General information, not medical advice. For anything about your own prescription or eye health, talk to an optician or optometrist.