1. What is Lens Effectivity Calculator?

Definition: This calculator determines the effective power of a lens at a different vertex distance than it was originally prescribed.

Purpose: It helps optometrists and opticians adjust lens prescriptions when the fitting distance changes.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( P_{eff} \) — Effective power (diopters)
• \( P \) — Lens power (diopters)
• \( d \) — Vertex distance (meters)

Explanation: The formula accounts for how lens power changes when moved closer to or farther from the eye.

3. Importance of Vertex Distance Compensation

Details: Proper vertex distance compensation ensures accurate prescription power reaches the patient's eye, especially important for high-power lenses.

4. Using the Calculator

Tips: Enter the lens power in diopters and vertex distance in meters (default 0.012m or 12mm). Vertex distance must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical vertex distance?
A: Standard vertex distance is 12-14mm (0.012-0.014m) for most eyeglasses.

Q2: When is vertex compensation most important?
A: For prescriptions above ±4.00 diopters, where small distance changes significantly affect effective power.

Q3: How does moving lenses closer affect power?
A: Moving plus lenses closer increases effective power; moving minus lenses closer decreases effective power.

Q4: What about contact lenses?
A: Contact lenses have zero vertex distance, so their power is the effective power at the corneal plane.

Q5: Why does the formula denominator include (1 - d×P)?
A: This accounts for the vergence change as light travels the vertex distance between lens and eye.