1. What is a Lens Effectivity Calculator?

Definition: This calculator determines the effective power of a lens at a different vertex distance (the distance between the lens and the eye).

Purpose: It helps optometrists, opticians, and eyewear professionals adjust prescriptions when changing frame styles that affect lens-to-eye distance.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

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

Explanation: As a lens moves closer to or farther from the eye, its effective power changes. This calculation accounts for that effect.

3. Importance of Lens Effectivity Calculation

Details: Proper calculation ensures patients receive the correct optical correction, especially important for high-power prescriptions.

4. Using the Calculator

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

5. Frequently Asked Questions (FAQ)

Q1: What is vertex distance?
A: The distance between the back surface of the lens and the front of the cornea, typically 12-14mm for glasses.

Q2: When is this calculation most important?
A: For high-power lenses (±4.00D or more) or when changing between glasses and contact lenses.

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's the typical vertex distance for glasses?
A: Standard is about 12mm (0.012m), but this varies by frame style and facial anatomy.

Q5: How accurate is this calculation?
A: Very accurate for thin lenses. For thick lenses, additional calculations may be needed.