Vertex Distance Conversion
The vertex distance is the distance between the back surface of a spectacle lens and the front surface of the cornea, typically 12-14 mm. When converting a spectacle prescription to a contact lens prescription, this distance must be accounted for because moving a lens closer to or farther from the eye changes its effective power. This is especially important for prescriptions above ±4.00 diopters.
For myopic (nearsighted) patients, the contact lens power is weaker (less negative) than the spectacle power because the lens is closer to the focal point. For hyperopic (farsighted) patients, the contact lens power is stronger (more positive) than the spectacle power. The difference becomes increasingly significant with higher prescriptions, reaching several diopters for very high myopia or hyperopia.
Formula
Where F is power in diopters and d is vertex distance in meters.
Conversion Table (12mm vertex)
| Spectacle (D) | Contact Lens (D) | Difference |
|---|---|---|
| -4.00 | -3.82 | +0.18 |
| -6.00 | -5.60 | +0.40 |
| -8.00 | -7.27 | +0.73 |
| -10.00 | -8.93 | +1.07 |
| +4.00 | +4.20 | +0.20 |
| +6.00 | +6.47 | +0.47 |
Clinical Significance
- For prescriptions below ±4.00 D, the vertex distance correction is negligible (less than 0.12 D).
- Vertex distance conversion is critical for high myopia patients transitioning from glasses to contacts.
- Inaccurate vertex distance assumptions can cause over- or under-correction of refractive error.
- Some trial frame measurements must be adjusted for vertex distance to compare with phoropter readings.
Frequently Asked Questions
When is vertex distance conversion clinically significant?
Vertex distance conversion becomes clinically meaningful (0.25 D or more difference) for spectacle powers above approximately ±4.00 D. For lower prescriptions, the difference is small enough that it falls within manufacturing tolerances and is generally ignored in practice. For high prescriptions (-10 D or more), the correction can exceed 1 diopter and is essential for accurate vision correction.
Why does moving a lens closer change its effective power?
A diverging (minus) lens placed closer to the eye intercepts the diverging light rays at a point where they have spread less. This means the effective vergence at the cornea is less negative than expected, requiring a weaker contact lens. The opposite occurs for plus lenses: moving them closer makes them effectively stronger because the converging rays have not yet converged as much.
What vertex distance should I use?
The standard assumed vertex distance is 12 mm for spectacle prescriptions, though actual distances range from 8 to 16 mm depending on frame style and face geometry. For the most accurate conversion, measure the actual vertex distance with a distometer or ruler. Contact lens vertex distance is 0 mm since the lens sits directly on the cornea.