How Vertex Distance Affects Contact Lens Power
Why effective power changes when the lens moves closer
Vertex distance is the space between the back surface of a spectacle lens and the front of the cornea. In a trial frame or phoropter, it is commonly about 12 to 14 mm. When you move a prescription from the spectacle plane to the corneal plane, the effective power at the eye changes.
For low prescriptions, the difference is usually smaller than a quarter-diopter step and often does not change the ordered contact lens power. Once meridional powers exceed roughly ±4.00 D, the effect can become clinically meaningful, especially in high myopia, high hyperopia, or anisometropia.
How vertex distance changes minus and plus lens powers
A simple way to keep the direction straight is to remember what happens when the lens gets closer to the cornea.
- Myopes (minus lenses): the contact lens power is usually less minus than the spectacle power at higher prescriptions. Example: a −6.00 D spectacle Rx often starts near −5.50 D as a contact lens power.
- Hyperopes (plus lenses): the contact lens power is usually more plus than the spectacle power at higher prescriptions. Example: a +6.00 D spectacle Rx often starts near +6.50 D as a contact lens power.
The calculator above applies the effective power formula directly. For a quick chairside reference without entering values, use the vertex distance conversion chart below.
Vertex distance conversion chart
This reference table shows approximate contact lens powers after vertex compensation at 12 mm for common spectacle powers. Use it as a quick lookup when you need a ballpark figure chairside, or to double-check your calculator result. The formula used is Fc = Fs ÷ (1 − d × Fs) with d = 0.012 m, rounded to the nearest 0.25 D step.
Minus powers (spectacle → contact lens at 12 mm vertex)
| Spectacle (D) | CL Power (D) | Change |
|---|---|---|
| -4.00 | -3.75 | +0.25 |
| -4.50 | -4.25 | +0.25 |
| -5.00 | -4.75 | +0.25 |
| -5.50 | -5.25 | +0.25 |
| -6.00 | -5.50 | +0.50 |
| -6.50 | -6.00 | +0.50 |
| -7.00 | -6.50 | +0.50 |
| -7.50 | -7.00 | +0.50 |
| -8.00 | -7.25 | +0.75 |
| -8.50 | -7.75 | +0.75 |
| -9.00 | -8.00 | +1.00 |
| -9.50 | -8.50 | +1.00 |
| -10.00 | -9.00 | +1.00 |
| -11.00 | -9.75 | +1.25 |
| -12.00 | -10.50 | +1.50 |
| -14.00 | -12.00 | +2.00 |
| -16.00 | -13.50 | +2.50 |
| -18.00 | -14.75 | +3.25 |
| -20.00 | -16.25 | +3.75 |
Plus powers (spectacle → contact lens at 12 mm vertex)
| Spectacle (D) | CL Power (D) | Change |
|---|---|---|
| +4.00 | +4.25 | +0.25 |
| +4.50 | +4.75 | +0.25 |
| +5.00 | +5.25 | +0.25 |
| +5.50 | +6.00 | +0.50 |
| +6.00 | +6.50 | +0.50 |
| +6.50 | +7.00 | +0.50 |
| +7.00 | +7.75 | +0.75 |
| +7.50 | +8.25 | +0.75 |
| +8.00 | +8.75 | +0.75 |
| +8.50 | +9.50 | +1.00 |
| +9.00 | +10.00 | +1.00 |
| +9.50 | +10.75 | +1.25 |
| +10.00 | +11.25 | +1.25 |
| +11.00 | +12.75 | +1.75 |
| +12.00 | +14.00 | +2.00 |
| +14.00 | +16.75 | +2.75 |
| +16.00 | +19.75 | +3.75 |
Values assume a vertex distance of 12 mm and are rounded to the nearest 0.25 D manufacturing step. For toric prescriptions, apply vertex compensation to each meridional power independently. Use the calculator above for exact values or non-standard vertex distances.
Toric prescriptions require meridional vertexing
Vertex distance applies independently to each principal meridian. For toric prescriptions, it is not enough to vertex only the sphere component. You need to consider both meridional powers.
Example: −5.00 −2.00 × 180 has meridional powers of −5.00 D and −7.00 D. Each meridian should be vertexed at the spectacle plane, then reconstructed back into sphere, cylinder, and axis at the corneal plane.
For routine prescribing, the Glasses to Contact Lens Calculator is usually the fastest workflow tool. Use this page when you want to isolate vertex distance, check a borderline case, or teach what is happening optically.
Back vertex distance and trial frames
The back vertex distance (BVD) is the measurement used in the vertex formula. It is measured from the back surface of the spectacle lens to the front of the cornea. Standard phoropter vertex distance is approximately 13.75 mm, and trial frames can vary from about 10 to 16 mm depending on fit and frame style.
When vertex distance is measured precisely (for example, with a distometer or a corneal reflection technique), you can enter the exact value in the calculator above. For routine prescribing, 12 mm is a common default that closely approximates most contact lens fitting scenarios.
What to do clinically after vertex compensation
Vertex-compensated power is a starting point. Final ordering still depends on lens design, fit, comfort, and an over-refraction on eye. When powers are high or binocular balance is sensitive, small changes can matter.
After vertexing, round to the nearest available manufacturing step and place the diagnostic or ordered lens on eye. An over-refraction with the Cross-Cylinder Calculator can refine the final power, especially in toric fits where rotation also plays a role.