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Glasses Prescription Explained: What the Numbers Mean

On a glasses prescription: OD = right eye, OS = left eye. Sphere (SPH) corrects nearsightedness (negative) or farsightedness (positive). Cylinder (CYL) and axis correct astigmatism. Add power applies to reading bifocals. A glasses prescription is not interchangeable with a contact lens prescription.

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What do OD and OS mean on a prescription?

These are Latin abbreviations:

  • OD (oculus dexter) — your right eye
  • OS (oculus sinister) — your left eye
  • OU (oculus uterque) — both eyes together (used for some measurements)

Prescriptions typically list OD in the top row and OS in the bottom. Some prescriptions use the plain-language labels "Right" and "Left" instead.

What does the sphere (SPH) number mean?

Sphere is the main corrective power of the lens, measured in diopters (D).

  • A negative number (e.g., -2.50) corrects myopia (nearsightedness) 3 — the eye focuses in front of the retina; the lens diverges light to push the focal point back.
  • A positive number (e.g., +1.75) corrects hyperopia (farsightedness) — the eye focuses behind the retina; the lens converges light to pull the focal point forward.
  • Plano (PL or 0.00) means no sphere correction is needed in that eye.

The magnitude of the number reflects how much correction is needed. A sphere of -0.50 is a mild prescription; -6.00 or higher is considered high myopia 1. The practical distinction matters for lens thickness — higher powers require thicker (or high-index) lenses.

What do cylinder (CYL) and axis mean?

These two fields work together and apply only if you have astigmatism — when the cornea or lens has an uneven curvature that causes light to focus at multiple points rather than one.

  • Cylinder (CYL): The additional power needed to correct the astigmatism, again in diopters. It is usually negative (or sometimes positive, depending on the optometrist's notation convention). A CYL value of 0.00 or blank means no astigmatism correction is needed.
  • Axis: A number from 1 to 180, measured in degrees. It describes the orientation of the cylinder correction — which meridian of the lens needs the astigmatic power. Axis alone is meaningless without the CYL; the two numbers define the full astigmatic correction together [1, 2].

If your prescription has no CYL entry, there is no astigmatism correction in that eye.

What is the "Add" power?

"Add" (addition) is the extra focusing power added to the lower portion of a bifocal or progressive lens to help with near vision. It is always a positive number (typically between +0.75 and +3.50) and usually the same for both eyes.

Add power is prescribed when presbyopia develops — typically starting in the early-to-mid forties — when the eye loses some of its natural ability to flex the lens for near focus. People with add powers are prescribed bifocals, progressive lenses, or separate reading glasses [1, 3].

What is the prism field, and do most people have one?

Prism corrects eye alignment issues (binocular vision problems), where the two eyes do not point at exactly the same place. It is measured in prism diopters and accompanied by a direction (base in, base out, base up, base down). Most prescriptions have no prism, and it requires specific clinical testing to determine whether prism is needed.

Why is a glasses prescription different from a contact lens prescription?

This surprises many people: a contact lens prescription is not the same as a glasses prescription and cannot be used interchangeably.

  • Vertex distance: Glasses sit about 12mm in front of the eye; contact lenses sit directly on the cornea. This changes the effective power of the lens. Moderate-to-high prescriptions must be converted mathematically for contacts.
  • Additional contact lens parameters: A contact lens prescription includes base curve (how curved the lens is, matched to your cornea shape) and diameter (size of the lens). These are critical for a safe and comfortable fit 2.
  • Brand-specific fitting: Different contact lens brands in the same power can fit differently. A contact lens prescription specifies the exact brand, power, base curve, and diameter that were verified as correct for your eyes.

Filling a contact lens order with glasses prescription numbers — or vice versa — can result in blurry vision or, with contacts, discomfort and potential corneal damage from an ill-fitting lens.

How often does a glasses prescription change?

Prescriptions tend to change most rapidly during childhood and adolescence as the eye grows. In most adults, the prescription stabilizes somewhat, though gradual changes throughout adulthood are normal, particularly in the add power as presbyopia progresses. High myopia can progress in adults as well.

The American Academy of Ophthalmology recommends comprehensive eye exams every one to two years for most adults, more frequently if you have risk factors for eye disease such as diabetes, glaucoma family history, or significant refractive error 2.

Common questions

Is -2.00 a bad prescription?

"Bad" is relative — -2.00 (two diopters of myopia) is a moderate prescription. Without glasses, you would likely see clearly up to about half a meter but find distant objects blurry. With the correct lenses, vision is fully corrected. Prescriptions of -6.00 or higher (high myopia) carry somewhat higher risk of certain retinal conditions over a lifetime, which is one reason regular dilated eye exams are important for high myopes.

Can I use my glasses prescription at any optical shop or online?

For glasses, yes — you are legally entitled to a copy of your glasses prescription and can fill it anywhere. For contact lenses, you must use the brand, base curve, and diameter specified by your eye doctor, as these are part of the fitting. Filling contact lenses with incorrect parameters can be unsafe.

What does it mean if my two eyes have very different prescriptions?

This is called anisometropia — a significant difference in prescription between the two eyes. It is not unusual and can often be fully corrected with appropriate lenses. Contact lenses are sometimes used because they eliminate the image-size difference that thick glasses lenses can create when prescriptions differ substantially.

My prescription says -1.75 -0.50 x 180. How do I read that?

That means: -1.75 diopters of sphere (mild nearsightedness), -0.50 diopters of cylinder (mild astigmatism), with the astigmatism correction oriented at 180 degrees. All three numbers are required to make that lens correctly.

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Important notes about prescription safety

  • —Never wear another person's glasses prescription — it will not correct your vision and can cause eye strain or headaches
  • —Contact lenses worn beyond their replacement schedule or with the wrong parameters can cause serious corneal infections
  • —Do not purchase prescription glasses online using approximate or self-tested numbers — an accurate in-office exam protects your vision

This article explains prescription terminology for educational purposes only. Your exact prescription must be determined by a licensed optometrist or ophthalmologist during a clinical eye examination. Gale can help you find an eye care provider and prepare questions for your visit.

References

  1. 1.Jacobs DS, Afshari NA, Bishop RJ, Keenan JD, Lee J, Shen TT, Vitale S; American Academy of Ophthalmology Preferred Practice Pattern Refractive Management/Intervention Panel (2023). Refractive Errors Preferred Practice Pattern. Ophthalmology. doi:10.1016/j.ophtha.2022.10.031 ✓Classification and correction of myopia, hyperopia, astigmatism, and presbyopia; definitions of prescription parameters
  2. 2.American Academy of Ophthalmology (2024). Eye Exam and Vision Testing Basics. American Academy of Ophthalmology EyeSmart Patient Education. link ✓Contact lens prescription requirements distinct from spectacle prescriptions; exam frequency recommendations for adults
  3. 3.National Eye Institute (2024). Refractive Errors — Eye Conditions and Diseases. National Eye Institute (NEI/NIH). link ✓Plain-language definitions of myopia, hyperopia, astigmatism, and presbyopia; correction principles that underlie glasses prescription fields

3 sources, numbered by first appearance. General health information, not medical advice — synthetic demonstration content.