If you've ever looked at your written eyeglass prescription and seen a wall of abbreviations and numbers, you're not alone. Here's a clear breakdown of what each part actually means, so you can order with confidence instead of just guessing that you've entered everything correctly.
OD, OS, and OU — Which Eye Is Which
These abbreviations come from Latin, and they're the first thing you'll see on most prescriptions:
- OD (Oculus Dexter) — your right eye
- OS (Oculus Sinister) — your left eye
- OU (Oculus Uterque) — both eyes together (used less often, usually only when a value applies equally to both)
Your prescription almost always lists OD and OS as separate rows, since most people have at least slightly different measurements between their two eyes.
SPH (Sphere)
This number represents the primary lens power needed to correct your distance vision, measured in diopters (D). It tells you the basic strength of correction, and whether you're nearsighted or farsighted:
- A minus number (like -2.50) means you're nearsighted (myopic) — you have trouble seeing things far away
- A plus number (like +1.75) means you're farsighted (hyperopic) — you have trouble seeing things up close
The further the number is from zero, in either direction, the stronger the correction needed.
CYL (Cylinder) and AXIS
These two values always appear together, and they only show up if you have astigmatism — a common condition where the cornea (the clear front surface of your eye) is shaped more like a football than a basketball, causing blurred or distorted vision at any distance.
- CYL indicates the additional lens power needed to correct that irregular shape, also measured in diopters. This number is usually written as negative, though some prescriptions use positive notation depending on the doctor's preferred format.
- AXIS is a number between 1 and 180, indicating the orientation of the astigmatism correction — essentially, at what angle the cylinder correction needs to be positioned in the lens.
If your prescription doesn't list CYL or AXIS at all, it typically means you don't have significant astigmatism, and your correction is purely spherical.
ADD (Addition)
This value only appears if you need additional magnification for close-up vision on top of your base prescription — most commonly due to presbyopia (the age-related loss of near-focusing ability, usually starting in your 40s). ADD is always a plus number, and it's added to your base SPH power specifically for the near-vision portion of a multifocal or progressive lens.
If your prescription includes an ADD value, that's a strong signal you need progressive or bifocal lenses, not single vision — see our earlier post on single vision vs. progressive lenses for a deeper breakdown of that distinction.
PD (Pupillary Distance)
PD is the distance between the centers of your pupils, measured in millimeters. Unlike the other values above, PD isn't always included on a written prescription by default — it's a separate physical measurement, and some doctors only provide it if you specifically ask. If yours isn't listed, you can measure it yourself at home (see our dedicated PD measurement guide) or ask your eye doctor's office directly, since most will provide it on request even after your exam.
Prism (If Present)
Not every prescription includes this, but if yours does, it indicates a correction for eye alignment issues — helping the eyes work together more comfortably if there's a tendency for one eye to drift relative to the other. Prism values are written with a specific direction (like "base up," "base in") alongside the numeric strength, measured in prism diopters. This is a more specialized correction, and if it's on your prescription, it's worth double-checking you've entered it exactly as written, since prism placement is precise.
Putting It All Together — Example
A typical prescription line might look like this:
| SPH | CYL | AXIS | ADD | |
|---|---|---|---|---|
| OD | -2.25 | -0.75 | 90 | +1.50 |
| OS | -2.00 | -0.50 | 85 | +1.50 |
Reading this: the right eye (OD) needs -2.25 diopters of base correction for nearsightedness, plus -0.75 diopters of astigmatism correction oriented at a 90-degree axis, plus an additional +1.50 diopters for near vision (indicating a progressive or bifocal lens). The left eye (OS) follows the same structure with slightly different values — which is completely normal, since eyes rarely match exactly.
A Few Things Worth Double-Checking
- Confirm your prescription is current. Most eyeglass prescriptions are valid for 1–2 years, depending on your state and your eye doctor's guidance — an expired prescription may not reflect your current vision needs.
- Make sure you're not confusing an eyeglass prescription with a contact lens prescription. These use different measurements and are not interchangeable — a contact lens prescription includes additional values (like base curve and diameter) that don't apply to eyeglasses, and eyeglass SPH values don't directly translate to contact lens power.
- If any part of your prescription is illegible or unclear, don't guess. Contact your eye doctor's office for clarification, or reach out to us — our reviewing optometrist checks every submission before production, and will flag anything that looks inconsistent or unclear before your lenses are made.
Have a prescription you're unsure how to read? Send it to hello@aldermiller.com before you order, and we'll walk through it with you.