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GUIDE

How to Read a Glasses Prescription: SPH, CYL, AXIS, ADD, PD

Written by Alvin · Manh Long Optical, 147A Le Duan, Cua Nam, Hanoi · Updated 11 September 2026

What a prescription looks like

Most prescriptions have two rows, one per eye, and a handful of number columns. Labels vary by where you were tested, some clinics use Vietnamese, some English, but the meaning is the same. The table below maps both:

OD and OS

These just mark which eye. OD is the right eye, OS is the left; sometimes shortened to R and L. When reading a prescription, always match the row to the correct eye, the two eyes are rarely identical.

SPH, the most important number

SPH is the eye's main correction. The sign decides which type:

• Minus (-): short-sightedness, or myopia. -2.00 means 2 diopters of myopia.
• Plus (+): long-sightedness, or hyperopia. +1.50 means 1.5 diopters of hyperopia.

The unit is diopters, in steps of 0.25. The higher the number, the stronger the correction. If the sphere box reads 0.00, Plano or PL, that eye has no spherical correction.

CYL and AXIS, always a pair

CYL is the astigmatism power. Astigmatism happens when the cornea's surface isn't perfectly round, so the image blurs or stretches in one direction. If there's astigmatism, the prescription will also show AXIS, a number from 0 to 180 giving the direction that correction is ground into the lens.

Read the two together: CYL -0.75 AXIS 180 means 0.75 diopters of astigmatism, set at axis 180. If the CYL box is blank or reads 0.00, there's no astigmatism, and the axis column stays blank too.

ADD, only on progressive and reading prescriptions

ADD is the extra power added for near vision, always a plus number. Anyone who struggles to read up close, usually from middle age onward, will have this figure. An ordinary distance-only lens has no ADD.

On a progressive lens, the top of the lens carries the distance power (per SPH), and the bottom adds ADD for near vision. If your prescription has an ADD figure, mention it when ordering so the right lens type gets chosen.

PD, pupillary distance, and why it matters when cutting a lens

PD is the distance between the centres of your two pupils, in millimetres, usually somewhere around 58 to 68. This number says nothing about your prescription, but it decides whether the glasses feel comfortable.

When a lens is cut, its optical centre needs to sit directly in front of the pupil. If PD is set wrong, you're looking through an off-centre part of the lens, which causes eye strain and headaches, especially with a strong prescription or a progressive lens. Many prescriptions don't include PD; when that happens, we measure it for you on the frame you've chosen.

Reading a sample prescription

Say a prescription reads:

PD: 63 mm

In words: the right eye is 2 diopters short-sighted, with 0.5 diopters of astigmatism at axis 170. The left eye is 2.5 diopters short-sighted, with no astigmatism. No ADD, so this is a distance-only single-vision prescription. Pupillary distance is 63mm. A half-diopter difference between the two eyes is normal.

What to bring when ordering glasses

Bring your prescription if you have one, and the glasses you're currently wearing. Your old glasses help the person testing your eyes know what prescription you're used to, and compare it against the new measurement.

If your prescription is over 12 months old, it's worth re-testing before cutting new lenses, since your prescription may have changed. We don't cut lenses from a prescription over 12 months old without re-testing. No prescription is fine too, an eye test in the shop is free. If you're not sure when you need glasses or how often to re-test, read how much myopia before you need glasses.

Common questions

What is SPH on a prescription?

SPH, also written as Sphere, is the eye's main correction. Minus is short-sighted, plus is long-sighted, in diopters, in steps of 0.25. For example -2.00 means 2 diopters of myopia.

What's the difference between a plus and a minus sign on a prescription?

A minus in the SPH column means short-sightedness, a plus means long-sightedness. In the ADD column on a progressive prescription, the number is always plus, since it's extra power added for near vision.

What is PD and why does it matter when cutting glasses?

PD is the distance between your two pupils, in millimetres. When cutting a lens, its optical centre has to line up with the centre of your eye; a wrong PD means you're looking through an off-centre part of the lens and strain more easily. We measure your PD for you when we cut your lenses.

How old can a prescription be before it needs re-testing?

If it's over 12 months old, re-test before cutting new lenses, since your prescription may have changed. We don't cut glasses from a prescription over 12 months old without a fresh test.