Glasses Prescription Vertex Distance Calculator
Use this calculator to adjust your spherical prescription power when changing the vertex distance, such as converting a glasses prescription for contact lenses, or vice-versa. This is particularly important for higher prescriptions.
Understanding Your Glasses Prescription and Vertex Distance
A glasses prescription is a precise measurement of the lens power needed to correct your vision. It typically includes several components, but for spherical correction, the 'Sphere' (SPH) value is key. This value, measured in diopters (D), indicates the degree of nearsightedness (myopia, negative values) or farsightedness (hyperopia, positive values).
What is Vertex Distance?
Vertex distance is the space between the back surface of your spectacle lens and the front surface of your cornea (the clear front part of your eye). For glasses, this distance is typically around 10-14 millimeters (mm). For contact lenses, the vertex distance is effectively 0 mm because the lens sits directly on the cornea.
Why is Vertex Distance Important?
The effective power of a lens changes with its distance from the eye. A lens with a certain power will have a different effect on your vision if it's held further away or closer to your eye. This effect becomes more significant with higher prescriptions (generally above +/- 4.00 diopters).
- For Nearsightedness (Myopia – negative SPH): If you move a minus lens closer to your eye (e.g., switching from glasses to contact lenses), its effective power becomes weaker (less negative). You'll typically need a slightly weaker (less negative) contact lens prescription.
- For Farsightedness (Hyperopia – positive SPH): If you move a plus lens closer to your eye, its effective power also becomes weaker (less positive). You'll typically need a slightly weaker (less positive) contact lens prescription.
Therefore, when switching between glasses and contact lenses, especially for higher prescriptions, your eye care professional will adjust the spherical power to compensate for this change in vertex distance. This calculator helps you understand that adjustment.
How to Use This Calculator
- Original Sphere Power: Enter the spherical power from your current prescription (e.g., -6.00, +3.50).
- Original Vertex Distance: Enter the vertex distance of your original prescription. For glasses, this is typically 10-14mm. For contact lenses, it's 0mm.
- Target Vertex Distance: Enter the vertex distance for the new type of vision correction. If converting from glasses to contacts, this would be 0mm. If converting from contacts to glasses, this would be 10-14mm.
- Click "Calculate Adjusted Power" to see the compensated spherical power.
Important Considerations
- This calculator only adjusts the spherical power. It does not account for cylinder (astigmatism), axis, or prism. These components are typically not adjusted for vertex distance in the same way or require more complex calculations.
- The results are for informational purposes only and should not replace a professional eye examination. Always consult your optometrist or ophthalmologist for your official prescription and contact lens fitting.
- Contact lens prescriptions also consider factors like base curve, diameter, and material, which are not covered by this calculation.
Example Calculation:
Let's say your glasses prescription is -8.00 Diopters, and the typical vertex distance for your glasses is 12mm. You want to know the equivalent contact lens prescription (vertex distance 0mm).
- Original Sphere Power: -8.00 D
- Original Vertex Distance: 12 mm
- Target Vertex Distance: 0 mm
Using the calculator, the adjusted power for contact lenses would be approximately -7.25 D. This shows that for high myopia, the contact lens power is weaker (less negative) than the glasses power.