How to Read Your Prescription
Eye Care
How to read your prescription
A prescription is a short table of numbers that almost nobody explains. Here is what each column means, so you can read your own.
The two rows
OD is your right eye and OS your left — oculus dexter and oculus sinister. Occasionally you will see OU, meaning both. Some prescriptions simply say R and L.
The columns
SPH — sphere
The main strength of the lens, in dioptres. A minus number corrects short-sightedness (myopia): distance is blurred. A plus number corrects long-sightedness (hyperopia). The further from zero, the stronger the lens — and the more the refractive index matters for thickness.
CYL and AXIS — astigmatism
If your cornea is shaped a little more like a rugby ball than a football, light focuses unevenly. CYL is how much correction that needs; AXIS is the orientation of it, from 1 to 180 degrees. The axis is not a strength — it is an angle, and it only has meaning when there is a CYL value beside it.
A blank CYL simply means no measurable astigmatism. It is not an omission.
ADD — the reading addition
Extra plus power for near work, added to the bottom of a bifocal or progressive lens. It usually appears once your early forties arrive and the eye's own focusing range starts to shorten. The same figure normally applies to both eyes.
PRISM and BASE
Uncommon, and only present when the eyes need help working together. Prism shifts the image; BASE says in which direction (in, out, up, down). If you have a prism value, the fitting of the frame matters more than usual.
PD — pupillary distance
The distance between the centres of your pupils, in millimetres. This is a fitting measurement rather than a correction, which is why it is often missing from a prescription written by someone who did not dispense the lenses. We measure it ourselves, in the frame you have chosen.
A worked example
| SPH | CYL | AXIS | ADD | |
| OD | −2.75 | −0.50 | 180 | +2.00 |
| OS | −3.00 | — | — | +2.00 |
Read aloud: moderately short-sighted in both eyes, very slightly more in the left. A small amount of astigmatism in the right eye only, oriented horizontally. A reading addition of +2.00 in both — so this person is a progressive or bifocal wearer. At around −3.00 a 1.60 index lens would sit comfortably in most frames.
What a prescription does not tell you
These numbers describe your eyes. They do not describe the lens you should buy. Two people with the identical table above may need entirely different lenses — different progressive corridors, different coatings, different centring — because one drives at night and the other spends nine hours at a monitor.
Nor does a prescription say anything about the health of your eyes. Pressure, retina and colour vision are checked in the examination, not written in the box. That is the part worth booking.
One honest note
A prescription older than two years is a starting point, not an instruction. Eyes change quietly, and the numbers do not age well. If yours has a date you cannot remember, bring it in and we will check it against what your eyes are doing now.