Contact Lens Over-Refraction Calculator
Recommended New Power
Understanding Contact Lens Over-Refraction
Over-refraction (OR) is a clinical technique used by optometrists and contact lens fitters to determine the final power required for a patient while they are already wearing a contact lens. It is the most accurate way to fine-tune a prescription because it accounts for the unique way a lens sits on the individual's cornea and the tear film layer between the lens and the eye.
How the Calculation Works
The math behind over-refraction involves more than just simple addition, especially when dealing with higher powers. The primary factor is Vertex Distance Compensation. When a patient is refracted behind a phoropter, the lenses are approximately 12mm to 14mm away from the eye. However, a contact lens sits directly on the eye (0mm vertex distance).
The formula used for the adjusted effective power is:
Where:
- F: The Over-Refraction power measured in Diopters.
- d: The vertex distance in meters (e.g., 0.012 for 12mm).
Example Calculation
Imagine a patient wearing a -6.00D contact lens. During the examination, the doctor finds an over-refraction of -1.00D at a 12mm vertex distance.
- Step 1: Adjust the -1.00D OR for vertex distance. Since -1.00 is low, the change is negligible (-0.99D).
- Step 2: Add the adjusted OR to the current lens. -6.00 + (-0.99) = -6.99D.
- Result: The practitioner would likely order a -7.00D lens.
When is this Calculator Useful?
- Annual Check-ups: When a patient's vision has slightly shifted but they are already wearing a trial lens.
- High Myopia/Hyperopia: When the power exceeds +/- 4.00D, vertex distance becomes crucial for clarity.
- Spherical Equivalent: Calculating the necessary sphere power when converting from a glasses prescription to a contact lens.
Medical Disclaimer: This calculator is intended for educational purposes for students and eye care professionals. Always consult with a licensed Optometrist or Ophthalmologist before ordering contact lenses.