An optical prescription is a table with about six columns and no explanation. Most people never learn to read theirs, which is fine right up until they want to order glasses online and cannot tell which number goes where.
This is a reading guide, not medical advice
Nothing here replaces an eye test. A prescription is issued by a qualified optometrist or ophthalmologist after examining your eyes, is typically valid for a limited period, and can change. If your vision has changed, book a test — do not interpret an old prescription yourself.
The two rows
Prescriptions are written in Latin abbreviations that predate the modern optical trade. OD (oculus dexter) is your right eye. OS (oculus sinister) is your left. Some clinics now write RE and LE instead, which is the same thing in plain language. OU means both eyes together.
The columns
| Abbreviation | Full name | What it does |
|---|---|---|
| SPH | Sphere | The main correction strength. A minus value corrects short-sightedness (myopia); a plus value corrects long-sightedness (hyperopia) |
| CYL | Cylinder | Extra correction for astigmatism — a cornea shaped more like a rugby ball than a football. Blank or 0.00 means none |
| AXIS | Axis | The orientation of that astigmatism correction, from 1 to 180 degrees. Only meaningful when CYL has a value |
| ADD | Addition | Extra magnifying power in the lower part of the lens, for reading. This is what makes a lens bifocal or progressive |
| PRISM / BASE | Prism | Rarely used. Shifts the image to help eyes that do not align perfectly |
Values are written in dioptres, in steps of 0.25. A SPH of −2.50 is a moderate short-sighted correction; −0.75 is mild; −7.00 is strong. The sign matters as much as the number, and a prescription written without one is incomplete.
PD: the number that is usually missing
Pupillary distance is the distance between the centres of your pupils in millimetres, and it decides where the optical centre of each lens is placed. Get it wrong by a few millimetres and lenses that are otherwise correct can feel subtly off — most noticeably with stronger prescriptions.
PD is a measurement, not a correction, so it is often left off the printed prescription. Your optician can give it to you on request, and is generally obliged to hand over the prescription they wrote. It is written either as a single number (around 54 to 74 for most adults) or as two, one per eye, when the two halves are not symmetrical.
What a lens order actually needs
- SPH for each eye, with its sign
- CYL and AXIS for each eye, if astigmatism is corrected — both, never one
- ADD, if you need reading correction in the same lens
- PD
- The date of the prescription and the issuing practice
Two things that are *not* interchangeable: a contact lens prescription and a glasses prescription. Contact lens values are measured against a lens sitting on the eye rather than 12mm in front of it, and they carry a base curve and diameter that glasses do not use. Ordering spectacle lenses from a contact prescription produces the wrong correction.
Frames and strong prescriptions
If your correction is strong, frame choice stops being purely cosmetic. Large lenses in a strong minus prescription get thick at the edges and heavy; smaller, rounder shapes keep both under control. Progressive lenses need enough lens height to fit the reading zone — check the lens height on the product page before falling in love with a very shallow frame.
Not sure whether a frame suits your prescription? Send us the details before you order and we will tell you plainly if it is a bad match.
Written by
Eyevora Editorial
Product & care desk at Eyevora. We stock the frames we write about, and we publish every measurement we hold. Spotted something wrong here? Tell us and we will correct it.
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- eye test