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Vertex Distance Calculator

Adjust effective power for vertex distance in high prescriptions

Vertex Distance Power Correction

Select a Starting Vertex Distance and Power Magnitude

Vertex Distance for Glasses to 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.

A practical clinical cue

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.

This calculator applies the effective power relationship directly so you can quantify the change for a given vertex distance instead of relying on an approximation.

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.

What to do clinically after the math

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.

Vertex Distance FAQs

At what power should I routinely apply vertex distance?

A common threshold is about ±4.00 D at the spectacle plane, using meridional powers for toric prescriptions. Below that, the effective power change is often less than 0.25 D and may not affect the ordered contact lens power. Above that, the difference can exceed available step sizes and affect acuity or binocular balance, especially in high myopia, high hyperopia, or anisometropia.

What formula does this calculator use?

The calculator uses the standard effective power relationship:
Fc = Fs / (1 − d · Fs)

Where Fc is the corneal plane power, Fs is the spectacle plane power, and d is the vertex distance in meters (for example, 0.012 for 12 mm).

For toric prescriptions, the relationship is applied to each meridian, then converted back into sphere, cylinder, and axis.

Why do high powers feel wrong if vertex distance is ignored?

In high myopia, moving a minus lens from the spectacle plane to the cornea increases its effective power. If you order the same nominal power as a contact lens, the patient can be over-minused. In high hyperopia, using the spectacle power without adjustment can leave the patient under-plussed at the corneal plane. Vertex compensation, then an on-eye over-refraction, makes the final ordered power more reproducible.

Should I vertex the sphere only, or the full toric prescription?

For toric prescriptions, vertex the meridional powers, not only the sphere. The cylinder means the two principal meridians can be far apart in power, and vertex distance can affect them differently. This is why the calculator treats each meridian separately before reconstructing sphere, cylinder, and axis at the corneal plane.