Vertex Distance Adjustment Calculator
Adjust lens power when changing the vertex distance (e.g., from glasses to contacts or between different frame positions).
About this calculator
A lens's effective power at the eye changes with how far it sits from the cornea, because the light-bending effect of a given diopter power is only exact at one specific distance. This calculator works in two steps: first it converts the Original Lens Power at the Original Vertex Distance into an equivalent power AT the corneal plane, using the formula P over (1 minus d times P), where d is the vertex distance in meters. It then re-projects that same corneal-plane power back out to whatever New Vertex Distance you specify, using the inverse of that formula. Setting New Vertex Distance to 0 mm gives the power a contact lens would need to match a spectacle prescription, since contacts sit directly on the eye.
This compensation only produces a meaningfully different number for stronger prescriptions -- industry guidance generally reserves vertex compensation for spectacle powers beyond roughly ±7.00 D and contact lens powers beyond roughly ±4.00 D, since the effect is small enough to ignore at lower powers. Adjusted Power is rounded to the nearest quarter diopter, matching how lenses are actually manufactured. Effective Power at Cornea is the unrounded intermediate value from step one, useful for understanding how much the vertex change is actually contributing before rounding takes over.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Inputs
Results
Adjusted Power
-4.75 D
How to Use This Calculator
- Enter the Original Lens Power in diopters (from the current glasses prescription or trial frame).
- Enter the Original Vertex Distance in mm — typically 12 mm for standard eyeglass frames.
- Enter the New Vertex Distance in mm — use 0 mm for contact lenses or the new frame's measured vertex distance.
- The formula P_new = P_effective / (1 + d₂ × P_effective) calculates the adjusted power.
- Review the Adjusted Power (rounded to the nearest 0.25 D) and Change Magnitude to determine whether a prescription update is clinically significant.
How the result changes with Original Lens Power
| Original Lens Power | Adjusted Power |
|---|---|
| -12 | -10.5 D |
| -7.5 | -7 D |
| -3.75 | -3.5 D |
| -2.5 | -2.5 D |
What each input means
- Original Lens Power
- Current lens power in diopters at the original vertex distance.
- Original Vertex Distance
- Original vertex distance in millimeters (glasses typically 12 mm).
- New Vertex Distance
- New vertex distance in millimeters (contacts = 0 mm, trial frame may vary).
How this is calculated
Formula
New Power = P / (1 - d × P)Worked example, using the default values
- Identify Input ParametersOriginal Lens Power = -5, Original Vertex Distance = 12, New Vertex Distance = 0 = 3 input(s) provided
- Calculate Adjusted Power-4.75 = -4.75
- Calculate Change MagnitudeChange Magnitude0.25 = 0.25
- Calculate Effective Power at Cornea-4.72 = -4.72
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does moving a lens from glasses to contacts require a power adjustment at all?
A lens's focal properties are defined relative to its own position, so moving the same physical lens power closer to or further from the eye changes where the focused image actually lands on the retina. Contacts sit at the corneal plane (0 mm vertex distance) while glasses typically sit about 12 mm in front of the eye, so an unadjusted spectacle power would focus light at the wrong point once worn as a contact lens.
Why does this compensation matter more for higher prescriptions?
The vertex-compensation formula's effect scales with the square of the lens power, so a small vertex distance change barely moves a low prescription but meaningfully shifts a strong one. That's why industry guidance generally only calls for compensation above roughly ±4.00 D for contact lenses and ±7.00 D for spectacles -- below those thresholds, the difference typically rounds away to nothing.
What does Effective Power at Cornea actually represent?
It's the intermediate result from converting Original Lens Power at its vertex distance into the equivalent power exactly at the corneal plane -- essentially what a contact lens or corneal-plane measurement would need to match the same optical effect. The final Adjusted Power then re-projects that corneal-plane value back out to whatever New Vertex Distance you're solving for.
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