How to Read Your Prescription
Eye Care
How to read your prescription
A prescription is a short table of numbers that looks more forbidding than it is. Here is what each column means — and which parts of it are missing, because no prescription contains everything a lens needs.
| Column | What it means |
|---|---|
| OD · RE | Right eye. Oculus dexter. |
| OS · LE | Left eye. Oculus sinister. |
| SPH | Sphere — the main correction, in dioptres. A minus figure is short-sightedness, a plus figure long-sightedness. The further from zero, the stronger the lens. |
| CYL | Cylinder — correction for astigmatism, where the eye is shaped slightly more like a rugby ball than a football. Blank or zero means none. |
| AXIS | The orientation of that astigmatism, from 1 to 180 degrees. Only appears when there is a CYL value. It is an angle, not a strength. |
| ADD | The extra reading power added at the bottom of the lens. Its presence is what makes a prescription bifocal or progressive. Usually the same in both eyes. |
| PRISM · BASE | Uncommon. Shifts the image to help eyes that are not aligning well together. Base gives the direction — in, out, up or down. |
| PD | Pupillary distance, in millimetres — how far apart your pupils are. Often absent, and it matters more than people realise. |
A worked example
RE −2.75 −0.50 175 ADD +2.00
LE −3.00 −0.75 010 ADD +2.00
Moderate short-sightedness, slightly stronger in the left eye, with a small amount of astigmatism in both. The reading addition of +2.00 tells us this person is over forty-five and wants progressives. The prescription is strong enough that a 1.60 index would be the sensible floor and 1.67 worth considering in a rimless frame.
None of which tells us where the corridor should sit on their face. That is the part we measure.
What your prescription does not contain
Three measurements are missing from every prescription ever written, because they depend on the frame rather than the eye:
Fitting height — how far up the lens your pupil sits in the frame you chose. Get this wrong on a progressive and the reading zone is somewhere you cannot use it.
Vertex distance — how far the lens sits from your eye. Changes the effective power on stronger prescriptions.
Pantoscopic tilt and wrap — the angles at which the frame sits. A wrapped sunglass frame needs the prescription compensated for them.
Practical questions
How old is too old?
If your prescription is more than a year or two old, have it checked before spending money on lenses. Eyes move, and a small change is the difference between comfortable and merely tolerable.
Can I use a contact lens prescription for spectacles?
No. Contact lenses sit on the eye and spectacle lenses sit twelve millimetres in front of it, so the powers are not interchangeable — particularly on stronger prescriptions. The two prescriptions are written separately for good reason.
Can I bring a prescription from elsewhere?
Yes, if it is recent. We will still take our own fitting measurements in the frame you have chosen, because no external prescription includes them.
I have the numbers but not the PD.
Come in and we will measure it in under a minute. Guessing it from a photograph, which some online retailers encourage, is where a great many badly-centred lenses come from.
Bring it in, or have it checked.