Calculate lens effectivity, lens power, or vertex distance from the other two values using diopters and mm, cm, or inches in eyewear.
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Lens Effectivity Formula
The calculator estimates effective power at a plane toward the eye from one lens-power meridian, using a thin-lens relationship. Vertex distance is nonnegative and converts to meters before calculation. This optical estimate is not a prescription or fitting recommendation.
- LE = lens effectivity, in diopters (D)
- LP = lens power, in diopters (D)
- VD_mm = vertex distance, in millimeters (mm)
- VD_m = vertex distance, in meters (m)
- 0.001 = conversion factor from millimeters to meters
In Effective power mode, the calculator finds the effective power from the entered lens power and vertex distance.
In Lens power mode, the calculator finds the lens power from the entered lens effectivity and vertex distance.
In Vertex distance mode, the calculator solves for the vertex distance needed to produce the entered relationship between lens power and lens effectivity.
Vertex Distance Reference Values
Vertex distance can be entered in millimeters, centimeters, or inches. The calculator converts all distance inputs to meters before applying the formula. Zero distance leaves power unchanged; zero powers do not determine a unique distance.
| Unit | Equivalent in millimeters | Example |
|---|---|---|
| 1 mm | 1 mm | 12 mm = 12 mm |
| 1 cm | 10 mm | 1.2 cm = 12 mm |
| 1 in | 25.4 mm | 0.4724 in ≈ 12 mm |
The effect of vertex distance becomes more noticeable as lens power increases, especially beyond about ±4.00 D.
| Lens power | Vertex distance | Calculated lens effectivity | General result |
|---|---|---|---|
| +4.00 D | 12 mm | +4.2017 D | Plus lens becomes effectively stronger |
| +8.00 D | 12 mm | +8.8496 D | Change is larger at higher power |
| -4.00 D | 12 mm | -3.8168 D | Minus lens becomes effectively weaker |
| -8.00 D | 12 mm | -7.2993 D | Change is larger at higher power |
Example Problems
Example 1: Calculate lens effectivity
You have a lens power of +6.00 D and a vertex distance of 12 mm.
The lens effectivity is +6.4655 D.
Example 2: Calculate vertex distance
You have a lens power of +6.00 D and a lens effectivity of +6.4655 D.
Convert meters to millimeters:
The vertex distance is approximately 12 mm. The rounded 6.4655 D input gives 11.9996 mm.
FAQ
What is lens effectivity?
Lens effectivity is the effective power of a lens after accounting for vertex distance. A lens does not have the same effective power at every distance from the eye. Moving a lens closer to or farther from the eye changes its effective optical power, especially for stronger prescriptions.
Why does vertex distance matter more for stronger lenses?
The formula multiplies vertex distance by lens power, so the effect grows as lens power increases. A 1 mm or 2 mm change may have little practical effect for a low-power lens, but it can produce a meaningful change for high plus or high minus lenses.
Why are plus and minus lenses affected differently?
Before the positive lens focal plane, increasing vertex distance makes plus lenses effectively stronger and minus lenses effectively weaker. This is why vertex distance is especially important when converting between spectacle lens power and an effective power at a different position.
