How to Read Your Prescription: The Ultimate Guide to Understanding Prescription Parameters

When you receive your eyeglasses prescription from an optometrist, it can feel a bit overwhelming with all the abbreviations, numbers, and symbols. Understanding these details is essential for choosing the right pair of glasses, especially if you're shopping online. In this guide, we’ll walk you through how to read your prescription and explain each parameter so you can confidently order your new glasses

Key Terms in an Eyeglass Prescription

1. OD and OS
   These are Latin abbreviations used in ophthalmology:
• OD (Oculus Dexter): Refers to your right eye.
• OS (Oculus Sinister): Refers to your left eye.
  Some prescriptions may also include OU (Oculus Uterque), which refers to both eyes.
2. Sphere (SPH)
   This number indicates the lens power needed to correct nearsightedness (myopia) or farsightedness (hyperopia).
• A minus (-) sign in front of the number indicates nearsightedness, meaning you have trouble seeing objects far away.
• A plus (+) sign indicates farsightedness, meaning you have difficulty focusing on close objects.
3. Cylinder (CYL)
   The cylinder number measures the degree of astigmatism in your eyes. Astigmatism is caused by an irregularly shaped cornea, which leads to blurred or distorted vision.
• A negative CYL value corrects nearsighted astigmatism, while a positive CYL value corrects farsighted astigmatism.
  If you don’t have astigmatism, this part of your prescription might be blank.
4. Axis
   The axis value is only present if you have astigmatism. It’s a number between 1 and 180 degrees that indicates the orientation of the astigmatism in your eye. This is important for aligning the corrective lenses properly in your glasses.
5. Add (Addition)
   If you need bifocal or progressive lenses, the Add section indicates the additional lens power required for near vision. This value is typically used for correcting presbyopia, a condition that makes it difficult to focus on close objects as you age.
• The add value is usually the same for both eyes and is indicated as a positive number.
6. Prism
   Prism correction is less common but may be included in your prescription if you have double vision (diplopia) or other binocular vision disorders. It helps align your eyes so they work together properly. The prism is measured in diopters and may have a directional notation such as:
• BU (base up)
• BD (base down)
• BI (base in)
• BO (base out)
7. Pupillary Distance (PD)
   This measurement indicates the distance between the centers of your pupils in millimeters. It ensures that the lenses are properly aligned with your eyes for optimal vision correction. Your PD can be one number (for both eyes) or two numbers if each eye has a different distance from the center of the nose.
• For example: PD 64mm or PD 32/31mm (32mm for the right eye and 31mm for the left eye).

Sample Prescription Format

In this sample prescription:

• The right eye (OD) has -2.00 diopters of nearsightedness, with -0.75 diopters of astigmatism correction at an axis of 120 degrees.
• The left eye (OS) has -1.50 diopters of nearsightedness, with -0.50 diopters of astigmatism correction at an axis of 90 degrees.
• Both eyes require an additional +1.50 diopters for near vision, as indicated in the Add column.
• There’s a prism correction of 0.50 diopters in the right eye, with the prism base oriented upward (BU).

Common Questions About Eyeglass Prescriptions

1. Can I use the same prescription for contact lenses and glasses?
   No, eyeglass and contact lens prescriptions are different. Contact lenses sit directly on your eyes, while glasses sit about 12mm from your eyes. This distance requires different lens powers.
2. Why is my prescription different for each eye?
   It’s common for one eye to have a different level of refractive error than the other. Your optometrist tailors the prescription to correct each eye’s vision individually.
3. What does it mean if there’s no CYL or Axis on my prescription?
   If there’s no number in the CYL and Axis columns, it means you don’t have astigmatism, or the astigmatism is so minor that it doesn’t need correction.
4. How often should I get my eyes checked?
   It’s recommended to have an eye exam at least once every two years, or annually if you wear glasses or contact lenses. Your prescription can change over time, so regular check-ups ensure you have the most accurate correction.

Conclusion

Understanding your eyeglass prescription is the first step to making sure you get the correct glasses for your vision needs. By learning the meaning behind each parameter, you can confidently choose the right lenses and frames to improve your vision and comfort. At HPGlasses, we make the process easy by offering personalized services, whether you're looking for prescription lenses, tinted lenses, or photochromic options.