Eye Care · Explainer

How to Read Your Eye Prescription: SPH, CYL, AXIS and PD Explained

Your prescription is a short set of codes. SPH is the main lens power, CYL and AXIS describe astigmatism, ADD is extra power for reading, and PD is the distance between your pupils. Here is what every abbreviation on the slip means, in plain language.

🕐 6 min read👓 Eye care📖 Explainer

How do you read an eye prescription?

Read your prescription one eye at a time, left to right across the row. Each eye has a main power called SPH, and if you have astigmatism it also has a CYL and an AXIS. A separate ADD value appears if you need help for reading, and your PD is the distance between your pupils. A minus sign means you are short-sighted, and a plus sign means you are long-sighted. Once you know those five terms, the whole slip makes sense.

OD and OS: right eye and left eye

Prescriptions often use Latin abbreviations for each eye. OD stands for oculus dexter, meaning your right eye. OS stands for oculus sinister, meaning your left eye. If you see OU, that means both eyes together. Some clinics simply write Right and Left instead. The right eye is usually listed first.

SPH: the sphere, or lens power

SPH, short for sphere, is the main strength of the lens, measured in units called dioptres. A minus value corrects short-sightedness, where distant objects look blurry. A plus value corrects long-sightedness, where near objects look blurry. The further the number is from zero, the stronger the correction. For example, minus 2.00 is a stronger short-sighted correction than minus 0.75.

CYL and AXIS: astigmatism

CYL, short for cylinder, appears only if you have astigmatism, where the eye is shaped more like a rugby ball than a sphere. It shows how much extra correction that irregular shape needs. AXIS always travels with CYL and is a number from 1 to 180 that describes the angle at which the correction sits. If the CYL box is blank, you do not have significant astigmatism and there will be no AXIS either.

ADD: the reading addition

ADD is extra magnifying power added for close work, and it appears when you need help reading, which usually develops with age. It is written as a plus value and is often the same for both eyes. ADD is what makes reading glasses, bifocals, and progressive lenses work, letting one pair serve both distance and near vision.

PD: your pupillary distance

PD, or pupillary distance, is the distance between the centres of your pupils, measured in millimetres. It ensures the optical centre of each lens sits directly in front of each eye, which keeps vision comfortable and sharp. Adult PD usually falls between 54 and 74 millimetres. It is essential when ordering power glasses online, so ask for it to be noted at your eye test.

Frequently asked questions