Lens Thicnkess Calculator

Refractive Index (n):
Use negative (-) for nearsightedness, positive (+) for farsightedness.
For Minus lenses: Min thickness is at the Center.
For Plus lenses: Min thickness is at the Edge.
Max Edge Thickness
--- millimeters
Center Thickness
--- millimeters
Lens Type
---

How it works?

This calculator uses the Sagitta (Sag) Approximation formula to estimate lens thickness. In ophthalmic optics, lenses are typically meniscus (curved), but the relationship between the front and back curves determines the thickness.

$$ Sag \approx \frac{r^2 \cdot P}{2000 \cdot (n - 1)} $$

Where:

  • r (radius): Half of the lens diameter (mm). Larger lenses require much greater thickness.
  • P (Power): The dioptric power of the lens (Absolute value).
  • n (Index): The refractive index of the lens material.

Calculating Total Thickness

For Minus Lenses (Concave): The center is the thinnest point.
$$ Edge = Center_{min} + Sag $$

For Plus Lenses (Convex): The edge is the thinnest point.
$$ Center = Edge_{min} + Sag $$

Note: This tool calculates the thickness at the thickest meridian. If you have Cylinder (Astigmatism) power, the edge thickness will vary around the lens. We display the maximum thickness, which determines if the lens will fit in the frame or look aesthetically pleasing.

Optimizing Lens Aesthetics

Lens thickness is the number one concern for patients with high prescriptions. Thick edges on minus lenses can create "coke bottle" effects, while thick centers on plus lenses create magnification and weight issues.

Factors Affecting Thickness

1. Refractive Index

The most effective way to reduce thickness. High index materials (1.67, 1.74) bend light more efficiently, requiring less curvature for the same power. Switching from CR-39 to 1.74 can reduce edge thickness by up to 50%.

2. Frame Selection

Frame size directly dictates the required "Lens Diameter." A larger frame requires a larger lens, which exponentially increases edge thickness (due to the r2 term in the formula).

3. Decentration

If your eyes are close together but you choose a wide frame, the optical center must be moved inward. This effectively increases the diameter of the lens needed on the temporal side, adding significant thickness.

4. Cylinder Axis

For astigmatism, the thickness varies around the edge. If the negative cylinder axis is at 180°, the thickest edges will be at the top and bottom. If at 90°, the thickest edges are at the sides.

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