How to read your eye prescription

The rows: OD, OS and OU

• OD, oculus dexter, is your right eye. Some slips write R or RE.
• OS, oculus sinister, is your left eye. Some slips write L or LE.
• OU, oculus uterque, means both eyes, usually used for ADD.

SPH, sphere

The main power of the lens, in dioptres (D), in steps of 0.25.

• A minus number means nearsighted (myopia): near things are sharp, far things are blurry. The bigger the number, the closer the blur starts. With −2.00, you see sharply only up to 1 ÷ 2 = 0.5 m, about arm's length.
• A plus number means farsighted (hyperopia): the eye has to focus even for distance, and near work gets tiring or blurry, more so with age.
• 0.00, PL or plano means no sphere correction for that eye.
• In China, Taiwan and Hong Kong, people say the number times 100 as dù(度): −2.00 is "200 度 of myopia".

CYL, cylinder

Extra power in one direction only. It corrects astigmatism, where the eye focuses one direction more strongly than the other, so blur is stretched and lights look like streaks.

• An empty box, DS (dioptre sphere) or SPH written in the CYL column means no astigmatism.
• Optometrists usually write CYL as a minus number, and many ophthalmologists write it as a plus number. Both describe the same lens. To convert, add CYL to SPH, flip the sign of CYL and turn the axis by 90°. −1.00 −0.50 × 180 is the same as −1.50 +0.50 × 90. The simulator accepts either.
• Up to about −0.75, cylinder is common and often left uncorrected. Whether yours needs correcting is a question for your optometrist.

AXIS

The direction of the cylinder, in degrees from 1 to 180. 90 is vertical, 180 is horizontal. It only appears when there is a CYL. The axis does not make the blur worse or better; it sets which way the blur is stretched. Change it in the simulator and you will see the streaks around street lamps turn.

ADD, addition

Extra plus power added to the bottom of bifocals or progressive lenses, or used for reading glasses. It usually appears after 40, when the eye's own focusing for near (accommodation) weakens; this is presbyopia. Typical values run from +0.75 to +3.00 and are normally the same for both eyes.

The other boxes

• PRISM and BASE: a prism shifts the image to help the eyes line up. Few prescriptions have one; the simulator does not show it.
• PD, pupillary distance: the distance between your pupils in millimetres, used to place the lens centres. Some slips leave it off and the optical shop measures it.
• Expiry date: prescriptions are usually valid for one or two years, depending on where you live.

A slip, decoded

• Both eyes are nearsighted. From the sphere alone, the right eye would be sharp up to 50 cm and the left up to 40 cm (in 度: 200 and 250). The cylinder adds blur at every distance.
• Both eyes have astigmatism, 1.25 D and 1.50 D, with the cylinder at 180 (horizontal). Blur from the cylinder is stretched vertically, so a street lamp at night becomes a vertical streak.
• No ADD: no reading power.

See this prescription in the simulator →

A glasses prescription is not a contact lens prescription

A contact lens prescription also lists the lens's base curve, diameter and brand, and its powers can differ from your glasses, because the lens sits on the eye instead of about 12 mm in front of it. The difference matters from about ±4.00 upward. Do not order contact lenses with a glasses prescription.

What the numbers do not tell you

A prescription describes the lens that corrects your focus. It says nothing about the health of your eyes. Only an eye exam can tell you that.

This page explains notation and simulates blur for learning. It is not an eye test or medical advice. Ask an optometrist or ophthalmologist about your eyes.

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