Contact Lens Vertex Calculator
Calculate contact lens power from spectacle prescription using vertex compensation: Fc = Fo / (1 - d × Fo)
Calculate Contact Lens Power
Spherical for simple myopia/hyperopia, Toric for astigmatism
Spectacle Prescription
Right Eye
Left Eye
Vertex Distance
Contact lens rests on cornea (0 distance)
Contact Lens Power Results
Right Eye Contact Lens
Sphere Power
Left Eye Contact Lens
Sphere Power
Calculation Details
Formula: Fcontact = Fspectacle / (1 - d × Fspectacle)
Vertex change (d): 12.0 mm = 0.0120 m
Rounding: Contact lenses are typically available in 0.25 D increments
Prescription Analysis
Prescription Examples
Vertex Distance Guide
Standard Distance
12-14 mm typical for most eyeglass frames
Contact Lens
Always 0 mm - lens rests on cornea
Measurement
Measure from back of lens to cornea
Key Concepts
Vertex Distance
Distance from lens back surface to cornea
Diopter (D)
Unit of lens power = 1/focal length (m)
Focal Power
Ability of lens to converge or diverge light
Applications
Spectacle to contact conversion
Frame adjustment calculations
Optometry practice
Precision lens fitting
Clinical research
Vision correction surgery
Understanding Contact Lens Vertex Compensation
What is Vertex Distance?
Vertex distance is the distance between the back surface of an eyeglass lens and the front surface of the cornea. This distance typically ranges from 12-14 mm for regular eyeglass frames. When switching to contact lenses, this distance becomes zero since the lens sits directly on the cornea.
Why Compensation is Needed
- •Changing vertex distance alters effective lens power
- •Same lens power at different distances creates different focal points
- •Contact lens must compensate to maintain clear vision
- •Higher powers require more significant compensation
Vertex Compensation Formula
Fc = Fo / (1 - d × Fo)
- Fc: Contact lens power (D)
- Fo: Original spectacle power (D)
- d: Vertex distance change (m)
Note: For contact lenses, d is always positive since the lens moves closer to the eye (from spectacle distance to zero distance).
Lens Types and Calculations
Spherical Lenses
Used for simple myopia (nearsightedness) or hyperopia (farsightedness). Single power calculation using the basic vertex compensation formula.
Toric Lenses
Used for astigmatism correction. Requires separate calculations for spherical power and cumulative power (sphere + cylinder), then derive cylinder power.
Clinical Considerations
Measurement Accuracy
Precise vertex distance measurement is crucial for high-power prescriptions where small changes significantly affect required contact lens power.
Market Availability
Contact lenses are typically available in 0.25 D increments. Round calculated values to nearest 0.25 D for practical fitting.
High Power Cases
Prescriptions above ±4.00 D show significant vertex compensation effects. Consider specialty lens designs for very high powers.