Art Optical Vertex Distance Calculator
Understanding Vertex Distance in Contact Lens Fitting
In optometry and ophthalmology, the vertex distance is the distance between the back surface of a corrective lens (glasses) and the front of the cornea. When transitioning from eyeglasses to contact lenses, this distance becomes zero because the lens rests directly on the eye.
This physical shift changes the effective optical power of the lens. For spectacle prescriptions greater than +/- 4.00 Diopters (D), this change is clinically significant and must be compensated for to ensure clear vision with contact lenses.
The Mathematics of the Vertex Compensation
The calculation uses the standard optical formula for effective power:
Fc = Fs / (1 – (d * Fs))
- Fc: Power of the contact lens (Diopters)
- Fs: Power of the spectacle lens (Diopters)
- d: Change in vertex distance in meters (Spectacle Vertex – Contact Lens Vertex)
Vertex Adjustment Rules of Thumb
While the Art Optical Vertex Calculator provides precision, practitioners often remember two key behaviors:
- Myopes (Nearsighted): As the lens moves closer to the eye (vertex decreases), the lens becomes effectively stronger. Therefore, less minus power is required in the contact lens.
- Hyperopes (Farsighted): As the lens moves closer to the eye, it becomes effectively weaker. Therefore, more plus power is required in the contact lens.
Example Calculation
If a patient has a spectacle prescription of -8.00D measured at a 12mm vertex distance:
- Spectacle Power (Fs): -8.00
- Distance (d): 0.012 meters
- Calculation: -8.00 / (1 – (0.012 * -8.00))
- Result: -7.30D
In clinical practice, this would typically be rounded to the nearest available contact lens power, which is -7.25D.