This tool adjusts lens power when the vertex distance changes. Default target is 0 mm, which approximates the contact lens plane.
Examples assume spectacle VD = 12 mm and target VD = 0 mm. Values are illustrative; rounding shown to 0.25 D.
| Case | Spectacle Rx (D) | VD Change | Converted Sphere | Converted Cylinder | Axis |
|---|---|---|---|---|---|
| 1 | -6.00 / -1.00 × 180 | 12→0 mm | -5.50 | -1.00 | 180° |
| 2 | +5.00 / 0.00 | 12→0 mm | +5.25 | 0.00 | — |
| 3 | -10.00 / -2.00 × 090 | 12→0 mm | -9.00 | -1.75 | 090° |
| 4 | -3.00 / -0.75 × 045 | 12→0 mm | -3.00 | -0.75 | 045° |
Vertex distance correction is based on vergence transformation between two planes.
- Enter spectacle vertex distance in millimeters.
- Set target distance to 0 mm for contacts.
- Fill sphere, cylinder, and axis for each eye.
- Select a rounding step that matches available lenses.
- Press calculate, then review converted and rounded outputs.
- Use the export buttons to save your results.
This calculator supports estimation only, not diagnosis.
The sections below explain when vertex correction matters and how to interpret the converted powers in real contact lens selection.
1) Why vertex distance changes lens power
Vertex distance is the separation between the back surface of a spectacle lens and the cornea. When that distance decreases, the effective vergence at the eye changes. Higher prescriptions shift more because the term d·F grows. For low powers, the difference can be clinically small.
2) Typical distances and what to measure
Many refractions assume about 12 mm as a default spectacle vertex distance, but real frames vary. Values of 10–14 mm are common, while wrap frames can be lower. Use a distometer when possible. For contact lenses, the target plane is often treated as 0 mm.
3) Useful data points at 12 mm to 0 mm
With a 12 mm change (d = 0.012 m), a −4.00 D meridian becomes about −3.82 D, a −6.00 D meridian becomes about −5.60 D, and a −10.00 D meridian becomes about −8.93 D. Positive lenses increase: +6.00 D becomes about +6.47 D.
4) Why the calculator converts meridians
Astigmatic prescriptions have two principal powers: S and S+C. Each meridian is vertex-adjusted first, then recombined to obtain a new sphere and cylinder. This keeps the cylinder accurate after conversion. The axis stays the same because only power changes with plane shift.
5) Rounding and available lens steps
Contact lenses are commonly stocked in 0.25 D steps for sphere and cylinder, although some ranges vary. After computing precise powers, rounding makes the result practical. If the rounded sphere or cylinder changes by more than 0.25 D from your spectacle Rx, re-check the vertex distance and confirm the refraction.
6) When vertex correction matters most
Many clinicians apply vertex correction around ±4.00 D and above. In the example above, the −6.00 D meridian changes by about +0.40 D, and the −10.00 D meridian changes by about +1.07 D. These shifts can noticeably affect acuity and comfort if ignored.
7) Interpreting spherical equivalent and cylinder
Spherical equivalent summarizes the average power (S + C/2), which helps compare prescriptions quickly, but it does not replace the full spherocyl correction. For toric contact lenses, prioritize the converted meridians and cylinder. Small SE changes may still hide meaningful cylinder changes in higher astigmatism.
8) Clinical checks after conversion
After selecting a lens based on vertex-adjusted power, verify over-refraction, lens fit, and vision in real wear. Tear film, corneal shape, and lens movement can alter effective power. Use this calculator as a planning aid, then confirm with a qualified eye-care professional before ordering or dispensing.
1) What is vertex distance in eyewear terms?
Vertex distance is the space between the back surface of your glasses lens and the front of your eye. It is usually measured in millimeters and affects how strong the lens feels at the eye.
2) Why convert a glasses prescription for contact lenses?
Contacts sit much closer to the cornea than glasses. For higher prescriptions, that plane change alters effective power. Converting helps choose a contact lens power that produces similar focus and clarity.
3) At what prescription should I care about vertex correction?
A common rule is around ±4.00 diopters and higher. Below that, changes are often small. Above that, ignoring vertex distance can lead to noticeable blur or over/under-correction.
4) Does axis change when vertex distance changes?
No. Vertex changes affect power, not orientation. The calculator keeps the same axis and adjusts the two principal meridians, then recombines them into a new sphere and cylinder.
5) Why does a minus prescription get “less minus” for contacts?
Moving a minus lens closer to the eye reduces its effective power at the cornea, so the contact lens typically needs less minus. The opposite happens with plus prescriptions, which become slightly stronger.
6) Should I round the converted result to 0.25 D?
Usually yes, because many lens parameters are manufactured in 0.25 D steps. If your brand offers different steps, match that. Always verify the final choice with an over-refraction.
7) Can this calculator replace a professional fitting?
No. It provides a mathematical estimate. Contact lens fit, material, corneal shape, and tear film all affect vision and comfort. Use the result as guidance and confirm with an eye-care professional.