Contact Lens Vertex Calculator

Moving a lens closer to the eye (glasses to contacts) changes the effective power.

How the Math Works

The Contact Lens Vertex Calculator uses the formula F_c = F ÷ (1 − d·F) to adjust spectacle prescriptions for contact lenses. Here, F is the spectacle power in diopters, d is the vertex distance (in meters) between the spectacle lens and the eye, and F_c is the resulting contact lens power. The formula accounts for the fact that contact lenses sit directly on the eye, reducing the effective optical distance. When the vertex distance (d) decreases (e.g., from glasses to contacts), the denominator (1 − d·F) becomes smaller, slightly increasing the power to maintain equivalent vision correction. This adjustment is critical for high-prescription lenses, where small distance changes significantly impact optical performance.

Practical Applications

To apply this calculation, input your current spectacle prescription (F) and the vertex distance (d) of your glasses frames. Vertex distance is typically measured in millimeters from the center of the lens to the eye, often ranging from 12–15 mm for most frames. Convert this distance to meters (e.g., 14 mm = 0.014 m) before using the formula. For example, a -5.00 D prescription with a 14 mm vertex distance yields F_c = -5.00 ÷ (1 - 0.014·-5.00) ≈ -4.67 D. This adjusted power ensures your contact lenses provide the same optical correction as your glasses, accounting for their closer proximity to the eye.

Day-to-Day Use

This calculator simplifies the transition from glasses to contact lenses, ensuring accurate vision correction without requiring a new eye exam. By adjusting the prescription for vertex distance, it prevents under- or over-correction that could cause blurry vision, headaches, or eye strain. Whether you're switching to contacts for sports, daily convenience, or lifestyle preferences, the tool helps you choose the right power confidently. It also aids in communicating precise measurements to eye care professionals, streamlining the fitting process and improving comfort during the adjustment period.

Worked example

−6 D at 12 mm → about −5.6 D as a contact lens.

FAQ