Patient Guide · 4 min read · Feb 2, 2026
You've received your prescription and it looks like a string of abbreviations and numbers. Here's a plain-English guide to each value — and why getting them precisely right makes the difference between glasses that work beautifully and ones that cause headaches.
In This Article
Your prescription uses OD and OS — Latin abbreviations for oculus dexter (right eye) and oculus sinister (left eye). Some prescriptions use RE and LE instead. The prescription for each eye may be completely different. OU (oculi uterque) refers to both eyes together and appears in measurements such as pupillary distance.
The sphere value corrects for nearsightedness or farsightedness. A negative number (e.g., −2.50) indicates myopia — you see better up close than at distance. A positive number (e.g., +1.75) indicates hyperopia — you have difficulty with near focus or experience strain trying to compensate. The larger the absolute value, the stronger the correction required.
Cylinder and axis always appear together. They correct astigmatism — a condition where the cornea or lens is not perfectly spherical. The cylinder value represents the power needed to correct the astigmatism; the axis (a number from 1 to 180) describes its orientation. Both values must be accurate. Even a small error in axis can cause distorted vision and eye strain.
The ADD value appears in prescriptions for bifocals, progressive lenses, or reading glasses. It represents additional magnifying power needed for up-close tasks — always a positive number, typically ranging from +0.75 to +3.50. ADD values increase gradually as presbyopia — the natural loss of near-focusing ability that begins in the early 40s — progresses.
Prism values appear in prescriptions for patients with binocular vision disorders or eye alignment problems. Prism bends light to reduce the effort the eyes must make to maintain single, comfortable vision. Prism correction requires specialized lab work and precise fitting.
These are not interchangeable — and the distinction is backed by federal law. The FDA classifies contact lenses as medical devices, which means a contact lens evaluation is legally required every year, separate from your routine eye exam. It can be performed the same day and is billed as a standard additional service that all optometry practices charge.
Beyond the legal requirement, the annual contact lens evaluation serves a critical health purpose: it checks that your lenses fit correctly, verifies your prescription, and screens for subtle corneal changes that can develop silently under contact lens wear. A contact lens prescription also includes parameters your eyeglass prescription doesn't: base curve, diameter, and lens-specific power adjustments unique to on-eye wear.
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