What do OD and OS mean on a prescription?

OD is oculus dexter, the right eye; OS is oculus sinister, the left eye. Many Indian prescriptions simply write R and L, or RE and LE, which mean the same thing. OU means both eyes together.

What does a minus SPH mean?

A minus sphere corrects myopia — short sight, where distant objects are blurred. A plus sphere corrects hypermetropia, long sight. The number is the power in dioptres, and the further it is from zero the stronger the lens.

Why is AXIS required when CYL is not zero?

CYL says how much cylindrical correction is needed and AXIS says at what orientation, from 1 to 180 degrees. A cylinder without an axis is meaningless because the correction has to be aligned. If CYL is zero the axis is irrelevant and normally left blank.

How do I convert a plus cylinder prescription to minus cylinder?

Add the sphere and cylinder to get the new sphere, flip the sign of the cylinder, and change the axis by 90 degrees — adding 90 if the axis is 90 or less, subtracting 90 if it is above 90. The two forms describe exactly the same lens.

How long is a spectacle prescription valid?

There is no single statutory expiry in India, but one year is the working convention for adults and shorter for children and for people with diabetes or a fast-changing power. If a prescription is more than a year old, the right answer is a fresh test, not a fresh lens.

How to read an eyeglass prescription

7 min read · Last updated 4 September 2026

A prescription is a compact instruction sheet for making a lens. Once you know what each column means it reads in seconds — and reading it correctly is the difference between a happy customer and a remake at your own cost. This guide covers every field you will meet at an Indian optical counter, including plus-to-minus transposition worked step by step.

1. The anatomy of a prescription

Almost every prescription is a small grid: two rows, one per eye, and a set of columns. The row labels are OD (right eye) and OS (left eye), or simply R and L. A typical one looks like this:

	SPH	CYL	AXIS	ADD	VA
R (OD)	−2.25	−0.75	180	+1.50	6/6
L (OS)	−2.00	−0.50	175	+1.50	6/6

Below the grid you will usually find PD, sometimes split into right and left, and often a note about the lens type wanted — single vision, bifocal or progressive — plus the tests date and the prescriber’s name and registration. Everything else on the sheet is commentary. Powers are written in steps of 0.25 dioptre, and a sign is never optional: a cylinder written as “0.75” with no sign is an incomplete prescription and should be clarified, not guessed.

2. SPH — the main power

SPH, the sphere, is the primary correction in dioptres. A minus value corrects myopia (short sight): the eye focuses in front of the retina, distance is blurred, and a concave lens moves the focus back. A plus value corrects hypermetropia (long sight) with a convex lens.

Zero, or the word plano, or a simple dash, all mean no spherical correction on that eye. The magnitude matters commercially as well as clinically: as the sphere gets stronger, an ordinary 1.50 index lens gets thicker and heavier, which is exactly the conversation the lens index guide is about.

A difference in power between the two eyes is anisometropia. Small amounts are common and unremarkable; from about 1.00 D it starts to matter, and beyond roughly 2.50 D — where LensVerge raises a warning — the thickness difference between the two lenses is visible and the customer should be told before the job is cut, not after.

3. CYL and AXIS — astigmatism

A perfectly spherical cornea focuses light to a single point. A cornea shaped a little more like the back of a spoon focuses it differently in different meridians, and that is astigmatism. CYL is how much extra correction is needed in one meridian, again in dioptres and again with a sign.

AXIS says where that correction sits, as an angle from 1 to 180 degrees measured anticlockwise from the horizontal. It is a direction, not a power, so it is never written with a sign and never with dioptres. If CYL has a value, AXIS is mandatory. If CYL is zero or blank, AXIS is meaningless and should be left blank rather than filled with a zero.

A misread axis is the single most common cause of “the new glasses feel wrong”. 180 and 018 are easy to confuse in handwriting, and so are 5 and 6 in a hurried 65 or 85. When a prescription is unclear, ring the prescriber. Re-cutting a pair of lenses costs more than the phone call.

4. ADD — reading power

ADD is the extra plus power needed for near work, and it appears once the eye’s own focusing range starts to shorten with age — presbyopia, which most people notice somewhere in their forties. It is always a plus number, it is almost always the same for both eyes, and it is added to the distance sphere to give the near power.

So a right eye of −2.25 with ADD +1.50 reads at −0.75. That arithmetic is why a customer with a moderate myopia often takes their glasses off to read: their eye is already doing what the reading segment would do.

ADD is what turns a single-vision job into a bifocal or progressive job, and it usually rises with age in 0.25 D steps up to around +3.00. If a prescription has an ADD but the customer wants single-vision distance glasses, that is a legitimate choice — record it, so nobody later thinks the ADD was missed.

5. PD — pupillary distance

PD is the distance between the centres of the pupils in millimetres, and it is what tells the lab where to put the optical centre of each lens. Get it wrong and you induce prism: the customer gets eye strain, headaches, sometimes double vision, and no amount of frame adjustment fixes it.

Distance PD — typically around 54 to 70 mm in adults, used for distance lenses.
Near PD — a few millimetres less, because the eyes converge for near work.
Monocular PD — written as two numbers, e.g. 32 / 31, measured from the bridge to each pupil separately. Faces are rarely symmetric, and monocular PD is essential for progressives.

PD is a dispensing measurement rather than a clinical one, so many prescriptions arrive without it. Measuring and recording it is the shop’s job — and recording it against the customer means the second pair, next year, does not need re-measuring.

6. Plus and minus cylinder, and transposition

The same lens can be written two ways. Ophthalmologists in India often prescribe in plus cylinder; optometrists and most lens labs work in minus cylinder. Neither is more correct. Transposition converts between them, and it has exactly three steps:

New sphere = old sphere + old cylinder (algebraically, signs included).
New cylinder = same number, opposite sign.
New axis = old axis ± 90. Add 90 if the axis is 90 or less; subtract 90 if it is above 90. The result always stays between 1 and 180.

Worked example

Take the plus-cylinder prescription +2.00 / +1.50 × 90.

New sphere: +2.00 + 1.50 = +3.50
New cylinder: +1.50 becomes −1.50
New axis: 90 is not above 90, so add 90 → 180

The transposed form is +3.50 / −1.50 × 180. It is the same lens, described from the other meridian. A second example the other way: −1.25 / −0.75 × 170 becomes sphere −2.00, cylinder +0.75, and axis 170 − 90 = 80, so −2.00 / +0.75 × 80.

Do the transposition once, write it down, and send the lab the form they work in. Doing it mentally at the counter and then again on the job card is how a sign gets flipped. Software that transposes for you and stores both forms removes the arithmetic entirely.

7. Visual acuity and expiry

VA is how well the eye sees, recorded with the correction in place. Indian prescriptions usually use the metric Snellen notation — 6/6 means the person reads at 6 metres what a normal eye reads at 6 metres; 6/9 or 6/12 is progressively worse. The American 20/20 is the same idea in feet. N6, N8 and so on describe near vision.

VA is a quality check, not a lens instruction. It tells you whether the correction achieved normal vision. A best-corrected VA well below 6/6 suggests something the lens cannot fix, and the appropriate response is a referral, not a stronger power.

On expiry: India has no single statutory validity period, but one year is the working convention for adults, and shorter for children, for anyone with diabetes, and for a power that has been moving. A prescription older than that should be re-tested rather than re-dispensed — which is exactly why a twelve-month recall system is both good practice and good business.

8. What the shop should record

Whatever arrives on paper, the shop’s own record should be complete and attached to the customer, not to the sale. Record:

SPH, CYL, AXIS and ADD for both eyes, in the form you will send to the lab
PD — distance and near, monocular where you have it
VA for both eyes, and any note the prescriber added
The prescriber’s name and the date of testing — not the date you billed
A photo or scan of the original sheet, so a disputed reading can be settled
The lens type and index actually dispensed, against that prescription

Two payoffs. When the customer returns with “same as last time”, you have last time. And when a remake is claimed, you can show precisely what was prescribed, what was ordered and what was fitted.

LensVerge stores prescriptions properly. Every field validated as it is typed — axis required when cylinder is non-zero, anisometropia flagged, plus/minus transposition built in — attached to the customer, printable and shareable on WhatsApp as a PDF, and printed on the A4 and A5 invoice alongside the frame photo snapshot, so the customer’s copy carries the power it was made to. See the billing software or start a 14-day free trial.