After an eye exam, your optometrist or ophthalmologist may write you a prescription for eyeglasses or contact lenses. The prescription will include several numbers and abbreviations. You may see the following abbreviations:

  • O.D.: oculus dexter (right eye)
  • O.S.: oculus sinister (left eye)
  • O.U.: oculus uterque (both eyes)
  • CYL: cylindrical correction, which is used to identify an astigmatism
  • AXIS: direction of an astigmatism correction
  • DV: distance vision, or the part of your prescription to help you see things far away
  • NV: near vision, or the part of your prescription to help you see things close-up
  • ADD: additional power measurement for bifocal and multifocal lenses

For each eye, the first number you’ll see after O.D., O.S., or O.U. is a spherical correction (SPH), measured in diopters. This number is used to identify how strong your lenses need to be to correct your vision.

If the number has a minus (-) sign next to it, it means you’re nearsighted. A plus (+) sign or no sign means you’re farsighted. A higher number, regardless of whether there is a plus or minus sign, means you’ll need a stronger prescription.

Similarly to the SPH, there will also be a number with a plus sign (for farsightedness) or a minus sign (for nearsightedness) that follows CYL. A higher number means that you have a more severe astigmatism.

A prescription for glasses is not the same as a prescription for contact lenses. That’s because glasses are positioned about 12 millimeters (mm) from your eyes, whereas contact lenses go directly on the surface of your eyes.

Both prescriptions will contain corrections for nearsightedness, farsightedness, and, if needed, astigmatism. A contact prescription will also include the following additional information:

  • Base curve. This is the curve of the inside of your contact lens. It’s usually between 8 and 10, and matches the curvature of your eye.
  • Diameter. This is the measurement from edge to edge of contact lenses, and is usually between 13 and 15 mm, depending on the size of your eye.
  • Lens brand or material. Your doctor may recommend a specific brand or type of contacts.
  • Expiration date. A contact prescription is usually only good for one to two years after it has been issued. After this date, you’ll need another vision test and new prescription to buy more contacts.

Astigmatism is a common vision condition that causes blurriness or distorted vision. It can affect the way light refracts on the retina.

On a prescription, this will be written as part of the cylindrical (CYL) correction.

If you see no number under CYL, it means you have no astigmatism, or the astigmatism is so slight you don’t need to correct it.

20/20 vision means you have normal visual acuity (or sharpness and clarity) at a distance of 20 feet away. It doesn’t mean perfect vision, though. It only means you can see clearly at a distance.

Overall visual ability also measures:

  • peripheral or side vision
  • eye coordination
  • depth of perception
  • focusing ability
  • color vision

20/15 vision is actually better than 20/20. Someone with 20/15 vision can see objects at 20 feet away that someone with 20/20 can only see at 15 feet away. The higher the second number, the less clear and sharp you’ll see objects at a distance.

Someone with 20/200 has some vision, but doesn’t see with the same clarity as people with a 20/100 or 20/40 vision.

Depending on your visual acuity, your ophthalmologist or optometrist will be able to determine if eyeglasses or contacts can help. In some cases, you may not be able to get 20/20 vision with corrective lenses, but you may be able to see more clearly than you would without glasses or contacts.

Vision doesn’t worsen as a result of getting older, but your risk for eye-related conditions and diseases does get increase as you age.

For example, you’re at a higher risk for the following after age 50:

For better eye health, you can try the following in addition to eating a healthy diet and exercising regularly:

  • wear sunglasses and a brimmed hat when you’re out in bright sunshine
  • wear protective eyewear when playing sports, or using power tools or chemicals
  • maintain a healthy blood pressure

Your prescription can change, so it’s important to get regular eye exams. Adults ages 19 to 40 with vision problems should get their eyes checked at least every two years. Adults older than 40 should get them checked once a year.

If you’re an adult with no vision problems, get your eyes checked every five years until age 30, and then at least every 2 to 4 years from age 40 to 65. After age 65, you’ll need regular eye exams at least every two years.

Let your doctor know if you notice any changes to your vision and need to be seen more often.

At checkups, your doctor will also screen for eye conditions like glaucoma, which can be treated if caught early.

Your vision prescription can change over time. It’s important to get regular eye checkups so your glasses and contact prescription stays up to date. Your eye doctor can also check for common eye conditions that may require additional treatment or correction.

If your vision changes or you’re having trouble seeing clearly, make an appointment to get your eyes checked and let your doctor know your symptoms.