How to read your eye prescription

Every prescription has a row for the right eye and a row for the left eye, and the two are almost never identical. In India you will see this written either way: OD and OS, the Latin oculus dexter and oculus sinister that most testing machines still print, or RE and LE, right eye and left eye, which is what many local prescriptions write out instead. Same two rows either way.

Across the top, the columns are usually SPH, CYL, axis and ADD, in that order. A plain machine print-out from the autorefractor looks a little denser than the trial-frame prescription you are actually given, because the final one has already been refined on your face and rounded to a number that is comfortable, not just correct on paper.

SPH, short for sphere, is the main corrective power, measured in dioptres and written to two decimal places. A minus number, like −1.25, corrects short sight (myopia): distant things are blurred and near things are fine. A plus number, like +1.50, corrects long sight (hyperopia): it can be the opposite, or it can mean the eye is basically healthy and just needs a little help focusing up close as it ages.

The size of the number is the strength of the correction, not a verdict on how bad your eyes are. A −1.00 and a −6.00 both mean short sight; the second eye simply needs more correction to see the same chart clearly. Going up by 0.25 at a time is normal as power is refined on the trial frame; going up by a full dioptre or more between two tests, especially within a year, is the kind of change worth mentioning when you come in next.

CYL, cylinder, is the amount of astigmatism: an eye that focuses light unevenly because the cornea is shaped a little more like a rugby ball than a football, sharp in one direction and soft in another. It is extremely common and has nothing to do with eye health; it is just a shape, corrected the same way short or long sight is.

Axis is the direction of that unevenness, written in degrees from 1 to 180. It only exists where there is a CYL, because there is nothing to point a direction at otherwise. A blank or 0.00 in the CYL column means no astigmatism was found, not a number you can ignore, and it is why that row has no axis next to it either.

ADD, addition, is extra plus power added on top of the distance power, for close work. It is what turns a distance prescription into a bifocal or a progressive, and it is almost always the same number for both eyes. It starts appearing around the early forties, as the natural lens inside the eye stiffens and near focus fades, a change called presbyopia that happens to everyone sooner or later, whether or not you already wear glasses for distance.

PD, pupillary distance, is the distance between the centres of your two pupils in millimetres. It is not a power and it does not change with your eyesight, but it decides where the optical centre of each lens sits in the frame. Get it wrong by a few millimetres and a strong lens in particular can cause a subtle pulling or eye strain that nothing on the prescription itself explains.

It is often measured separately with a ruler or a pupillometer at the time the frame is chosen, rather than printed by the autorefractor, which is why it is sometimes missing from a photographed prescription. Most adults fall somewhere between about 54 and 74 millimetres; children measure less. If you are ordering glasses anywhere that is not in front of you, this is the one number worth double-checking before the lenses are cut.

Take the example above: the right eye (OD) reads SPH −1.25, CYL −0.50, axis 180, ADD +1.50. That is a mild degree of short sight, a small amount of astigmatism running exactly horizontal (axis 180), plus reading power for a presbyopic patient. The left eye (OS) reads −1.00, −0.75, 170, +1.50: slightly less short sight, a touch more astigmatism, the axis a little off horizontal, and the same reading addition as the other eye, which is normal since ADD rarely differs between the two.

Nothing about that pair of numbers is unusual. Most adult prescriptions in their forties and fifties look roughly like this: a small SPH, a small CYL, and an ADD that climbs gradually over the following years. What matters is not memorising the ranges but recognising your own numbers well enough to notice when one of them moves.