To ensure you order the right lenses, you should understand what the abbreviations and numbers mean on your eyeglass Rx. It can be helpful to familiarize yourself with the various symbols and terms because your prescription card may look different depending on your particular eye doctor.

Your eyeglass prescription could include the following details, so it is beneficial to know more about each term and common abbreviations:

OS and OD

When you read your eyeglass Rx, you will likely see the abbreviations “OS” and “OD.” These are short for oculus sinister (OS), meaning left eye, and oculus dexter (OD), meaning right eye. You may also see oculus uterque (OU), meaning both eyes.

Nearsighted and Farsighted

“Nearsighted” and “farsighted” are other terms you may come across regarding your eyeglass prescription. You will likely see them symbolized as either a plus (+) or a minus (-) sign.

If your Rx includes a minus symbol, it indicates you are nearsighted. Nearsightedness is a very common vision condition that causes objects that are at a distance to appear blurry and out of focus. It is also known as myopia.

The opposite of nearsightedness is farsightedness. This is another common condition that causes objects that are up close to appear unclear. If you are farsighted, your eyeglass prescription will include a plus sign. You may also hear the term “presbyopia” for this condition.


Those with astigmatism have irregularly shaped eyes that are less rounded and more comparable to the shape of a football. This condition can cause visual blurriness due to the extra light that enters the eye. Because the light is then refracted in the wrong direction, it can create glares and impact an individual’s ability to see clearly, both close up and at a distance.

Astigmatism can develop over time or be present from birth, depending on the person. If you have astigmatism, your eyeglass prescription will include cylinder and axis numbers that indicate how your lenses will correct the blurriness caused by your eye shape.

Sphere, Cylinder and Axis

Sphere is typically abbreviated as SPH and is an indication of your required lens power. You will notice a plus (+) or minus (-) symbol on your prescription for SPH. Again, the plus sign marks that the prescription is for correcting farsightedness, while the minus indicates it is for correcting nearsightedness.

The cylinder (CYL) number on your eyeglass Rx indicates lens power for correcting astigmatism. This number represents the difference between the weakest and strongest powers of your eyes. Those without astigmatism will have nothing noted for CYL.

If you do have astigmatism, you will also have an axis number that denotes the position of astigmatism in your eyes and the angle of the lens that will not include a cylinder power for correction.

Optical Power

The main numbers included on an eyeglass prescription express the optical powers of each lens. This value is also known as:

  • Dioptric power
  • Refractive power
  • Focusing power
  • Convergence power

The further away from zero this number is, the more powerful the Rx will be. You could see whole numbers or decimals, depending on your unique visual needs.

Pupillary Distance

You may also see pupillary distance (PD) on your prescription. Binocular PD is the measurement between the center point of each pupil. Monocular PD is the distance from your nose’s center to your pupil.

These figures can help to ensure your lenses are aligned with your eyes and centered properly for the best vision correction.


Individuals needing visual correction for various distances may see the term “ADD” on their eyeglass prescriptions. Short for “addition,” ADD involves the inclusion of a secondary lens power for multifocal and progressive lenses.

Those who need multiple lens powers often experience vision changes related to age, such as presbyopia.

BD, BU, BI and BO

If you are getting prism lenses, some other abbreviations and values you may see on your eyeglass Rx include:

  • Base down (BD)
  • Base up (BU)
  • Base in (BI)
  • Base out (BO)

These letters are used to indicate the right positioning of prisms for corrective lenses that fix double vision.