Contact Lens Power Converter — Complete User Guide
Clinical methods for converting spectacle prescriptions to contact lens (corneal plane) power, including vertex distance concept, formula derivation, and high prescription examples.
contact lens conversionvertex distancecorneal planecontact lens fittingspectacle prescription
Why is Conversion Needed?
There is approximately 12mm distance (called vertex distance) between spectacle lens and cornea. Contact lenses sit directly on the cornea without this distance.
The same refractive error requires different lens powers for these two types due to different distances from the corneal plane.
Conversion Formula
Dcontact = Dspectacle / (1 − d × Dspectacle)
Where d is vertex distance in meters, e.g., 12mm = 0.012m.
Examples
- Spectacle −6.00D (12mm) → Contact: −6.00 / (1 − 0.012 × −6.00) = −6.00 / 1.072 = −5.60D
- Spectacle +4.00D (12mm) → Contact: +4.00 / (1 − 0.012 × 4.00) = +4.00 / 0.952 = +4.20D
Clinical Recommendations
- Myopia ≥ −4.00D or Hyperopia ≥ +3.00D: Conversion recommended
- Higher prescriptions have greater vertex distance impact: −10.00D can differ by more than 1D
- Spectacle prescription ≠ contact lens power; please use this tool for conversion
References
- Phillips AJ, et al. Contact Lenses. 6th ed. Elsevier, 2020.
- Grosvenor T. Primary Care Optometry. 5th ed. Butterworth-Heinemann, 2007.