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Eyeglass Prescription Guidelines
Definition of Eyeglass Prescription Terms

Common terms used on spectacle/eyewear/eyeglass prescriptions have the follow meaning:

  • Pupillary Distance (PD): Pupillary Distance (PD) or inter-pupillary distance (IPD) is the distance (the industry standard is to measure in millimeters) between the centers of the pupils in each eye. A typical PD value is between 46 and 74. It is needed to ensure the center of each lens is aligned with the center of your pupils. Positioning lenses correctly in relation to the center of the pupils is especially important for higher-powered lenses due to the location of the optical center of the lenses. We would need your PD before we can place your order for prescription lenses. This is NOT part of your prescription and should be preferably obtained from your optician. If you do not have it now, please email that to us ASAP.
  • Sphere (SPH): The sphere refers to the amount and type of prescription you require. Short-sight is corrected by negative (-) sphere values, long sight by positive (+). If the sphere value for one eye is negative, then the other is usually also negative and vice versa. If you only see one number for each eye on your prescription, it’s the “SPHERE” power. It is common for the doctor to leave out the decimal points. A number like -25 or +175 is the same as -0.25 or +1.75, respectively.
  • Cylinder (CYL): The cylinder is only relevant if you have an astigmatism (or visual distortion). This value determines the strength of the astigmatism correction. If the cylinder value for one eye is negative, then the other is usually also negative, and vice versa. Prescriptions can be written in + cylinder construction (ophthalmologists write it this way) or – cylinder construction (the rest of the optical world writes in).
  • Axis (AXI)– The axis value is the angle of the astigmatism correction. If you have a cylinder reading on your prescription, then you should have an axis reading. An axis of 5 (or 05) is the same as 005; 90 is the same as 090. 
  • Reading Addition (ADD): The addition value is the amount of additional correction to add to the sphere value for reading or intermediate glasses. If you only see one ADD power, it’s understood that you have the same ADD for both eyes.

Pupillary Distance (PD): Your Pupillary distance (PD) is the distance, in millimeters, from one pupil to the other. A correct PD will ensure that the lens optical center (the point at which light enters the lens undistorted) is placed directly at your pupil. PD measurements are often taken individually for each eye.

Additional Eyeglass Prescription Abbreviations and Terms

Spectacle/eyewear/eyeglass prescriptions frequently contain several abbreviations and terms e.g.:

  • DV or Distance Vision: This specifies the part of the prescription designed primarily to improve far vision. In a bifocal lens, this generally indicates what is to be placed in the top segment.
  • NV or Near Vision: This may represent a single-vision lens prescription to improve near work, or the reading portion of a bifocal lens. Some prescription forms use ADD in place of NV with a single box to indicate the additional refractive power to be added to the spherical of each eye.
  • OD or Oculus Dexter: Latin for right eye from the patient’s point of view. Oculus means eye. 
  • OS or Oculus Sinister: Latin for left eye from the patient’s point of view.
  • OU or Oculi Uterque: Latin for both eyes.
  • Spherical Correction: A spherical correction corrects refractive error of the eye with a single convergent or divergent refractive power in all meridians.
  • Cylindrical Correction: A cylindrical correction corrects astigmatic refractive error of the eye by adding or subtracting power cylindrically in a meridian specified by the prescribed axis. 
  • Axis: The axis indicates the angle in degrees of one of two major meridians the prescribed cylindrical power is in. Which major meridian is referenced is indicated by the cylindrical correction being in plus or minus notation. The axis is measured on an imaginary semicircle with a horizontal baseline that starts with zero degrees in the 3 o’clock (or east) direction and increases to 180 degrees in a counterclockwise direction.