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Contact Lens Conversion Calculator
Convert spectacle prescriptions to contact lens power using vertex distance formulas and calculate spherical equivalents.
Spectacle to Contact Lens Converter
Vertex conversion with spherical equivalent calculation
Clinical Notes
- Vertex conversion is most significant for powers ≥ ±4.00 D
- Spherical equivalent is useful for soft lens fitting
- Standard vertex distance is 12mm for spectacles
- Results rounded to nearest 0.25 D
Clinical Reference
When to Use Vertex Conversion
Vertex conversion becomes clinically significant for powers of ±4.00 D or greater. For lower powers, the difference is typically less than 0.25 D and may not be clinically meaningful.
Spherical Equivalent Uses
- • Soft spherical lens fitting for low astigmatism
- • Initial lens selection before over-refraction
- • When cylinder is ≤0.75 D, spherical equivalent often works well
Standard Vertex Distances
- • Spectacles: 12-14 mm (commonly 12 mm)
- • Trial frame: 12-14 mm
- • Phoropter: 13.75 mm typical
- • Contact lens: 0 mm (at corneal plane)