Glasses prescription and myopia: how to read sphere, cylinder and axis?

OD, OS, sphere, cylinder, axis, numbers preceded by a minus sign: an eyeglasses prescription is not always easy to decipher. If you are short-sighted and interested in refractive surgery, knowing how to read your glasses prescription helps you understand your own correction and follow the ophthalmic surgeon's explanations more easily. Here is what each entry means, box by box, and what a prescription can, or cannot, tell you before myopia surgery.

The columns of a glasses prescription

A glasses prescription has one line per eye. On French prescriptions you will read OD for the right eye and OG for the left eye; on English documents, the American Academy of Ophthalmology (AAO) notes that you will find O.D. (oculus dexter) and O.S. (oculus sinister).

For each eye, you will usually find:

• the sphere, which corrects myopia or hyperopia;
• the cylinder and the axis, which correct any astigmatism;
• sometimes an add (addition), used only for near vision.

Sphere, cylinder and add are expressed in diopters (D), the unit of lens power. The axis is expressed in degrees.

The sphere: the myopia number

The sphere is the lens power needed to see clearly. According to the AAO, a minus sign (−) in front of this number indicates myopia, and a plus sign (+) hyperopia. The further the number is from zero, the stronger the correction.

For myopia, the definitions of the International Myopia Institute use a threshold of −0.50 D for myopia, and −6.00 D for high myopia. What these numbers mean inside the eye is explained in the guide What is myopia?

Cylinder and axis: astigmatism

The cylinder shows the amount of astigmatism. The AAO describes it this way: the cornea does not have the same curvature in every direction, a bit like an American football rather than a round ball. If the box is empty, no astigmatism correction has been prescribed.

The axis does not measure strength: it gives the orientation of the astigmatism, in degrees, from 1 to 180 according to the AAO. The cylinder may be written with a minus sign or a plus sign depending on the prescriber's habits: both notations can describe the same correction. Myopia and astigmatism often go together; their laser correction is covered in the article Astigmatism: laser correction at the same time as myopia.

Add, visual acuity, contact lenses: the other entries

• The add (Add): extra power for near vision, prescribed when presbyopia appears, for example for progressive or bifocal lenses (AAO).
• Visual acuity (expressed in tenths in France): it does not appear in the correction boxes. It measures what you can read on the letter chart, with or without correction. Sphere and cylinder, on the other hand, measure the correction. The two are linked, since good vision without glasses requires low sphere and cylinder values (AAO), but they are two different measurements.
• The contact lens prescription: it is not the same as the glasses prescription, because the lens sits directly on the eye; it also includes measurements specific to the lens (AAO).

The tests used to measure myopia are described on the page Myopia: symptoms and diagnosis.

A sample prescription, line by line

Here is a fictitious prescription, given only to learn how to read one:

• OD: −3.00 (−0.75 × 170°): the right eye is short-sighted (sphere −3.00 D) and has astigmatism (cylinder −0.75 D) oriented at 170 degrees;
• OG (left eye): −2.50: the left eye is short-sighted (sphere −2.50 D), with no astigmatism correction prescribed.

It is common for the two eyes to have different corrections. This example says nothing about whether surgery is possible: it only helps you identify each value.

Your prescription before myopia surgery

Your prescription gives a first idea of your correction, but it is not enough to consider an operation. During the pre-operative assessment, the refraction is measured again precisely, after dilating the pupil if needed, and completed by other tests: shape and thickness of the cornea, optical quality of the eye, ocular surface and retina.

If you have kept your successive prescriptions, they show how your correction has changed over the years. The stability of myopia is one of the things the ophthalmic surgeon examines, as explained in the article Am I a candidate for myopia surgery? If you wear contact lenses, also remember to stop them before the assessment: see when to stop your contact lenses.

The numbers on a prescription cannot, on their own, tell whether surgery is possible or which technique would be suitable. Dr Gozlan, ophthalmic surgeon, determines the technique after the assessment, based on all the tests; the main ones are presented on the page Techniques. The aim of refractive surgery is to reduce dependence on glasses and contact lenses, with no guarantee of results.

FAQ: reading a glasses prescription when you are short-sighted

Further reading

• What is myopia?: the guide to understanding myopia;
• Symptoms and diagnosis: how myopia is measured;
• Myopia: solutions and treatments: glasses, contact lenses and surgery;
• Pre-operative assessment: the tests carried out before surgery;
• What is refractive surgery?: principles, techniques and steps;
• Astigmatism: laser correction at the same time as myopia.

Sources

• American Academy of Ophthalmology. How to Read an Eyeglasses Prescription. EyeSmart. aao.org
• American Academy of Ophthalmology. What are "normal" results of an eye exam? Ask an Ophthalmologist. aao.org
• Flitcroft DI, He M, Jonas JB, et al. IMI – Defining and Classifying Myopia: A Proposed Set of Standards for Clinical and Epidemiologic Studies. Invest Ophthalmol Vis Sci. 2019;60(3):M20-M30. PMID 30817826