What Is Vertex Distance?
Vertex distance is the distance from the back surface of a spectacle lens to the front of the cornea. When a lens is moved closer to or farther from the eye, its effective power at the corneal plane changes. This is why spectacle and contact lens prescriptions differ for moderate-to-high powers.
Vertex Compensation Formula
Fₛ is spectacle power (D), Fᶜₗ is contact lens power (D), and d is vertex distance in metres. The first formula converts spectacles to CLs; the second converts CLs to spectacles.
When Is Conversion Needed?
Below ±4.00 D, the difference is typically < 0.25 D and is absorbed by the rounding step. Above ±4.00 D, the difference becomes clinically meaningful. At −10.00 D, it is about 1.09 D; at −20.00 D, about 3.90 D.
Clinical Considerations
This calculator provides approximate optical conversions only. Always confirm with an eye care professional. Factors not captured include: tear lens, lens base curve, corneal curvature, pupil size, and accommodation. This tool is for educational reference, not clinical prescription dispensing.
How to Use
- Select spectacle-to-CL, CL-to-spectacle, or a table mode.
- Enter power and vertex distance.
- Click Calculate for converted power with clinical rounding.
Examples
−6.00 D specs, 12 mm vertex
F_CL = −6.00 / (1 − 0.012 × (−6)) = −5.58 D → −5.50 D
+8.00 D specs, 12 mm vertex
F_CL = +8.00 / (1 − 0.012 × 8) = +8.85 D → +9.00 D
−15.00 D specs, 12 mm vertex
F_CL = −15.00 / (1 − 0.012 × (−15)) = −12.71 D → −12.75 D
FAQ
What is vertex distance?›
Vertex distance is the distance from the back surface of a spectacle lens to the front surface of the cornea. The standard value is 12 mm, but it can range from 10 to 15 mm depending on frame fit.
Why do spectacle and contact lens powers differ?›
A spectacle lens sits farther from the eye, so its effective power at the corneal plane differs. Moving the lens closer (to contact lens position) changes its effective power. For minus lenses, the CL power is less negative; for plus lenses, it is more positive.
When is vertex conversion necessary?›
Clinically, vertex conversion becomes significant for powers above +4.00 D or below −4.00 D. Below this range, the difference is less than 0.25 D and is typically ignored.
What does rounding to 0.25 D mean?›
Contact lenses are manufactured in 0.25 D steps (e.g., −5.50, −5.75, −6.00). The calculated CL power must be rounded to the nearest available increment.
How does vertex distance affect high prescriptions?›
At −15.00 D with d = 12 mm, the CL power is approximately −12.71 D — a difference of 2.29 D. This demonstrates why conversion is critical for high prescriptions.
Can I use this for astigmatism?›
This tool handles the spherical component. For toric (astigmatic) prescriptions, each meridian is converted separately, and the vertex distance effect is smaller on the cylinder component.
Sources
Author & technical reviewer
Manish Kumar
PhysicsCalcs tools are reviewed with an educational focus: clear formulas, transparent assumptions, and practical context for students and science learners.
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