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Educational content only
This page is general patient education, not medical advice. It does not diagnose conditions, recommend specific treatments for you, or replace a conversation with your eye care provider. Always consult a qualified clinician before making decisions about your eye health.
Your prescription has rows of letters, numbers, and plus or minus signs that probably mean nothing on first read. Here's what each column actually says — and why your optician asks for some measurements that aren't on the slip.
Column by column
What each value does — and what's missing.
The shape
Sphere (SPH)
Negative numbers (-1.00, -2.50) correct nearsightedness. Positive numbers (+1.50, +2.75) correct farsightedness. The bigger the number, the stronger the correction. Each 0.25 is a small step.
The distortion
Cylinder + Axis (CYL × AXIS)
Astigmatism — a cornea that's more football-shaped than basketball-shaped. Cylinder is the magnitude (often -0.50 to -3.00), axis is the angle (1-180 degrees) where the correction sits. Both numbers travel together.
The reading boost
ADD power
An extra plus power added for near tasks, used in bifocals, progressives, and readers. Common values: +1.00 to +3.50. Appears only when presbyopia is part of your prescription. Both eyes usually get the same ADD.
The fitting bit
Pupillary distance (PD)
How far apart your pupils sit, in millimeters. Adult range: 54-74mm. Critical for centering your prescription — a 3mm error in a progressive can put the wrong zone in front of your pupil. Ask your optical to measure or read it from a frame fitting.
Get a fresh exam if
Your prescription is more than 1-2 years old, you're newly noticing eye strain, or one eye seems significantly stronger than the other. An old Rx isn't dangerous, but you're missing comfort and clarity.
Common questions
Honest answers to common questions.
Why is my PD not on my prescription?+
By federal rule, an eye exam Rx doesn't have to include PD — it's considered a fitting measurement, not part of the medical prescription. Many offices give it on request; some online retailers can measure it from a phone photo.
What does '-3.50 sphere' mean about my vision?+
It tells you the lens power needed to correct your eye. Roughly, -1.00 is mild nearsightedness, -3.00 is moderate, -6.00 and above is high. Most adults sit between -1.00 and -4.00. The number doesn't translate to a 20/something visual acuity.
What if my prescription shows DS or sphere only?+
DS means "diopter sphere" — no astigmatism correction needed. The cylinder and axis columns may be blank or marked SPH/DS. Means you only need a spherical correction.
Why are my contact lens and glasses prescriptions different?+
They sit in different places — contacts on the eye, glasses about 12mm in front. For prescriptions over about ±4.00, the math shifts. Contacts also include base curve and diameter values that glasses don't need.
My OD and OS numbers are very different — is that bad?+
Big differences (more than 1.50 diopters between eyes) is called anisometropia. It's fittable but can be hard to wear in glasses at first. Contacts often work better. If it's new or growing, your eye doctor should investigate.