If you’re thinking about switching from contact lenses to glasses, it’s easier than you might think. For low prescriptions (below ±4.00D), the difference between contact lens and glasses prescriptions is usually negligible, so you can use the same prescription. However, for higher prescriptions, adjustments to the lens power are necessary.
Why Are Contact Lenses and Glasses Prescriptions Different?
The primary difference between contact lenses and glasses prescriptions lies in the distance between the corrective lens and the eye:
Vertex Distance: Glasses are typically positioned 12-14 millimeters in front of the eyes, while contact lenses sit directly on the cornea. This difference in distance significantly impacts the required lens power, especially for those with higher prescriptions.
Lens Shape and Design: Contact lenses are designed to match the curvature of the eye, while glasses are flat and sit some distance away from the face. This difference in shape also contributes to the variation in prescriptions.
Understanding Your Contact Lenses and Glasses Prescription
Contact Lenses Prescription Components:
Base Curve (BC): The curvature of the lens, designed to fit the shape of your eye.
Diameter (DIA): The overall size of the lens.
Sphere (PWR or SPH): The lens power needed to correct your vision, measured in diopters (D). This is the primary value that needs to be converted.
Cylinder (CYL) and Axis: For those with astigmatism, these values correct the irregular curvature of the cornea.
Glasses Prescription Components:
Sphere (SPH): The power needed to correct nearsightedness or farsightedness.
Cylinder (CYL): Corrects astigmatism, if present in your prescription.
Axis: Indicates the direction of astigmatism, ranging from 0 to 180 degrees. It doesn’t represent power but rather the specific angle where astigmatism is corrected.
ADD: Mainly used for presbyopia patients, indicating the additional power needed for reading at close range.
Pupillary Distance (PD): The distance between the centers of the pupils in millimeters.
Converting Your Contact Lenses to Glasses Prescription
There is a conversion formula between contact lens prescription and glasses lens prescription:
For prescriptions over ±4.00D, here is a simple conversion guide:
- For glasses prescriptions between 4.00D to 5.00D, subtract 0.25D for nearsightedness and add 0.25D for farsightedness when converting to contact lenses.
- For glasses prescriptions between 5.00D to 7.00D, subtract 0.50D for nearsightedness and add 0.50D for farsightedness.
- For glasses prescriptions between 7.00D to 9.00D, subtract 0.75D for nearsightedness and add 0.75D for farsightedness.
- For glasses prescriptions between 9.00D to 11.00D, subtract 1.00D for nearsightedness and add 1.00D for farsightedness.
Here’s a table to help you understand how to convert contact lenss to glasses prescription quickly.
| Nearsightedness (-) | Farsightedness (+) | ||
| Contact lens powers | Glasses powers | Contact lens powers | Glasses powers |
| -4.00 | -4.25 | +4.50 | +4.25 |
| -4.25 | -4.50 | +4.75 | +4.50 |
| -4.50 | -4.75 | +5.00 | +4.75 |
| -4.75 | -5.00 | +5.25 | +5.00 |
| -4.75 | -5.25 | +5.75 | +5.25 |
| -5.00 | -5.50 | +6.00 | +5.50 |
| -5.25 | -5.75 | +6.25 | +5.75 |
| -5.50 | -6.00 | +6.50 | +6.00 |
| -5.75 | -6.25 | +6.75 | +6.25 |
| -6.00 | -6.50 | +7.00 | +6.50 |
| -6.25 | -6.75 | +7.25 | +6.75 |
| -6.50 | -7.00 | +7.50 | +7.00 |
| -6.50 | -7.25 | +8.00 | +7.25 |
| -6.75 | -7.50 | +8.25 | +7.50 |
| -7.00 | -7.75 | +8.50 | +7.75 |
| -7.25 | -8.00 | +8.75 | +8.00 |
| -7.50 | -8.25 | +9.00 | +8.25 |
| -7.75 | -8.50 | +9.25 | +8.50 |
| -8.00 | -8.75 | +9.50 | +8.75 |
| -8.00 | -9.00 | +9.75 | +9.00 |
| -8.25 | -9.25 | +10.25 | +9.25 |
| -8.50 | -9.50 | +10.50 | +9.50 |
| -8.75 | -9.75 | +10.75 | +9.75 |
| -9.00 | -10.00 | +11.00 | +10.00 |
Prescription Conversion Example
Suppose your contact lens prescription is:
- Sphere (SPH): -8.00D
- Cylinder (CYL): -1.50D
- Axis: 180
After applying the conversion formula:
Converted SPH: -8.75D
CYL and Axis: These values might remain unchanged or be slightly adjusted during the eye exam.
The final glasses prescription might be:
- SPH: -8.75D
- CYL: -1.25D
- Axis: 180
Important Considerations
- Prescription Validity: Both contact lens and glasses prescriptions have an expiration date, typically one or two years from the last eye exam. Ensure your prescription is still valid before making the conversion.
- Comfort and Fit: Even with the correct prescription, some individuals might experience discomfort or vision problems when switching from contact lenses to glasses or vice versa.
- Different Brands and Lens Types: The brand and type of lens (e.g., multifocal, toric lenses) can also affect the prescription conversion.
Conclusion
Converting a contact lens prescription to a glasses prescription involves understanding how lenses correct vision differently at various distances. Although the process may seem complex, following these steps and consulting with an eye care professional ensures that your glasses provide accurate vision correction. Whether you’re switching temporarily or permanently from contact lenses to glasses, having the correct prescription is key to maintaining clear and comfortable vision.
