Vertex conversion shifts lens power between two planes. To convert a spectacle-plane power F (diopters) at vertex distance d (meters) to the corneal plane:
For astigmatism, we apply the formula to both principal meridians (axis and axis+90°), then rebuild sphere/cylinder in your chosen notation.
| Spectacle Sphere (D) | Cylinder (D) | Axis (°) | Vertex (mm) | Approx. CL Sphere (D) | Approx. CL Cylinder (D) |
|---|---|---|---|---|---|
| -6.00 | 0.00 | — | 12 | -5.60 | — |
| -10.00 | 0.00 | — | 12 | -8.93 | — |
| +8.00 | 0.00 | — | 12 | +9.09 | — |
| -8.00 | -1.50 | 180 | 12 | -7.21 | -1.36 |
Spectacle lenses sit in front of the eye, while contact lenses rest near the cornea. The distance between these planes changes effective power. Clinically, the difference becomes meaningful around ±4.00 D and increases as power rises.
Many refraction lanes assume a vertex distance near 12 mm, but real frames vary. Common practical ranges are 10–14 mm, with higher plus and higher minus prescriptions being the most sensitive to vertex changes. This calculator lets you test 10, 12, or 14 mm instantly.
For negative powers, the contact lens power is usually less minus than the spectacle power. Example at 12 mm: −10.00 D spectacles convert to about −8.93 D at the corneal plane. That shift can reduce over‑minusing during contact lens fitting.
For positive powers, the contact lens power typically increases. Example at 12 mm: +8.00 D spectacles convert to roughly +9.09 D at the corneal plane. Small vertex errors can noticeably change plus fitting outcomes for aphakia or high hyperopia.
A spherocyl prescription has two key meridians: the axis meridian and the meridian 90° away. This tool converts both meridian powers using the same vertex formula, then rebuilds sphere and cylinder. That approach keeps the optics consistent when cylinder is strong.
Many lens catalogs use 0.25 D steps, while some specialty ranges offer finer increments. Try 0.25 D for typical ordering, 0.12 D for finer estimates, or 0.01 D to view raw math. Rounding can slightly change cylinder magnitude and the final sphere value.
Optical labs may present cylinder in minus or plus notation. When changing notation, sphere and cylinder values shift and the axis rotates by 90°. The calculator rebuilds the prescription in the notation you choose, so the final output matches the format you need.
Use vertex conversion as a starting point, not a final prescription. Verify with lens fit, comfort, and over‑refraction. As a quick data check, −6.00 D at 12 mm converts to about −5.60 D, and −8.00/−1.50×180 converts near −7.21/−1.36×180.
It is most useful for prescriptions stronger than about ±4.00 D. The higher the power, the larger the change between spectacle and contact lens planes, especially if vertex distance differs from 12 mm.
Use 12 mm as a common default. If you suspect a tight or loose frame fit, compare 10 mm and 14 mm outputs to understand how sensitive the result is for your prescription.
The axis stays the same during vertex conversion. However, if you switch between minus and plus cylinder notation, the axis transposes by 90° as part of the notation change.
Moving a negative lens closer to the eye reduces the effective minus power needed at the cornea. That is why contact lens powers often look “weaker” than spectacle powers for high myopia.
Moving a positive lens closer to the eye increases the effective plus power required at the cornea. This effect becomes noticeable in higher hyperopia and aphakic corrections.
Not fully. With astigmatism, each meridian may change differently. Converting both principal meridians and rebuilding sphere/cylinder gives a better estimate, especially when cylinder or sphere is high.
No. The output is an educational estimate. Final contact lens selection depends on lens design availability, base curve, fit, movement, tear film, and over‑refraction. Always confirm clinically with an eye‑care professional.