Handle high powers with accurate vertex conversion. Choose direction, rounding, and cylinder notation instantly here. Clear steps guide you from input to output smoothly.
Vertex conversion moves lens power between two planes. Use vertex distance d in meters and power F in diopters.
For spherocyl prescriptions, the calculator converts both principal meridians: M1 = Sphere and M2 = Sphere + Cylinder, then rebuilds the result.
| Direction | Vertex (mm) | Sphere (D) | Cyl (D) | Axis | Example output (approx.) |
|---|---|---|---|---|---|
| Spectacles → Contact | 12 | -10.00 | 0.00 | — | -8.93 DS |
| Spectacles → Contact | 12 | +8.00 | 0.00 | — | +8.89 DS |
| Spectacles → Contact | 13 | -6.00 | -2.00 | 180 | ≈ -5.56 / -1.73 × 180 |
| Contact → Spectacles | 12 | -8.00 | 0.00 | — | -8.83 DS |
| Contact → Spectacles | 12 | +10.00 | +1.00 | 090 | ≈ +8.93 / +0.90 × 090 |
Examples are approximate and depend on rounding choices.
Vertex distance is the space between a spectacle lens and the eye’s cornea. Typical values are 10–14 mm, but even a 2 mm change can shift strong prescriptions. This calculator uses vertex distance d in meters and converts power F in diopters. For cylinder prescriptions, it converts both principal meridians and rebuilds the result in the cylinder format you choose. The effect is small near plano, but beyond about ±4.00 D the difference becomes more noticeable, especially above ±8.00 D where shifts near 0.75–1.25 D are common.
At −2.00 D, adjustments are usually minimal. At −10.00 D with a 12 mm vertex, the contact equivalent is about −8.93 D, a difference of roughly 1.07 D. For +8.00 D at 12 mm, the contact equivalent is about +8.89 D, nearly +0.89 D higher.
Select Spectacles → Contact lens when starting from a frame prescription. Select Contact lens → Spectacles when estimating spectacle power from an existing lens power. The sign changes in the denominator because the reference plane moves in opposite directions.
A spherocyl prescription has two principal powers: M1 = Sphere and M2 = Sphere + Cylinder. Each meridian is vertex-adjusted independently, then the calculator reconstructs sphere and cylinder while keeping the same axis value for the final Rx.
Many lens powers are ordered in 0.25 D steps, so rounding is practical. Use “Nearest” for general ordering, “Up” to bias slightly stronger, or “Down” to bias slightly weaker. Use 0.125 D if your inventory supports it, or keep 0.01 D for exact math.
Spherical equivalent (SE) is Sphere + Cylinder/2. It helps you confirm overall trend when cylinder is present. The calculator shows SE for the input and the exact output so you can compare how the overall power balance changes.
Changing vertex distance increases the shift at high powers. Moving from 10 mm to 14 mm can change the converted result by around 0.2–0.4 D near −8.00 D, depending on the meridian. Enter the best estimate from your measurements or fitting setup.
Compare exact and rounded outputs, then verify that meridian values shift consistently. If cylinder rounds to 0.00, the display switches to DS. Use the export buttons to save results as CSV for logs or PDF for sharing calculations with your team.
1) What is vertex distance in simple terms?
It is the distance from the spectacle lens to the cornea. Moving a lens closer or farther changes effective power, especially for stronger prescriptions.
2) Why do higher prescriptions change more?
The vertex formula includes d·F. As power F increases, the adjustment grows nonlinearly, so high plus or minus values shift more than low values.
3) Does axis change after conversion?
No. The axis is a direction, not a power. This calculator converts the two meridians and keeps the same axis when rebuilding the output prescription.
4) Should I force plus- or minus-cylinder output?
Use the format that matches your ordering or record system. The optical result is equivalent; only the representation changes. “Same as input” is a safe default.
5) What rounding step should I choose?
0.25 D is common for many lenses. Use 0.125 D if your product range supports it. Use 0.01 D when you want exact math without rounding.
6) Are the example outputs exact?
They are approximate because results depend on vertex distance, cylinder format, and rounding preferences. Run the calculator with your exact values for a precise output.
7) Can this replace clinical fitting decisions?
It supports calculations, not medical decisions. Fitting also depends on base curve, diameter, material, and ocular measurements. Use the tool as a calculation aid.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.