What Is Vertex Distance?
Vertex distance is the distance from the back surface of a spectacle lens to the front surface of the cornea. In clinical practice, this distance is typically 12 to 14mm and is measured using a distometer or estimated during refraction. While this distance may seem small, it has a significant effect on the effective power of the lens, particularly for higher prescriptions.
When a patient transitions from spectacles to contact lenses, the lens moves from 12-14mm in front of the eye to directly on the corneal surface (effectively 0mm vertex distance). This change in position alters the effective power of the lens, requiring a mathematical adjustment to maintain the same visual correction.
When Is Conversion Necessary?
Vertex distance conversion becomes clinically significant when the spectacle prescription exceeds +/-4.00 diopters. Below this threshold, the difference between spectacle and contact lens power is negligible (typically less than 0.25 D) and can be safely ignored.
The higher the prescription, the more significant the vertex distance effect becomes. A -10.00 D spectacle prescription, for example, requires a substantially different contact lens power than the spectacle value.
The Conversion Formula
The formula for converting a spectacle prescription to contact lens power is:
Fcl = Fspec / (1 - d x Fspec)
Where:
- Fcl = contact lens power (what you are calculating)
- Fspec = spectacle lens power (from the refraction)
- d = vertex distance in meters (not millimeters)
The vertex distance must be converted from millimeters to meters before using the formula. For a standard 12mm vertex distance: d = 0.012 meters.
Worked Examples
Example 1: Minus Prescription
A patient has a spectacle prescription of -8.00 D with a vertex distance of 12mm.
Fcl = -8.00 / (1 - (0.012 x -8.00))
Fcl = -8.00 / (1 - (-0.096))
Fcl = -8.00 / 1.096
Fcl = -7.30 D (rounded to nearest 0.25: -7.25 D)
The contact lens requires less minus power than the spectacle lens. This makes intuitive sense because moving a minus lens closer to the eye increases its effective power, so you need less actual power to achieve the same correction.
Example 2: Plus Prescription
A patient has a spectacle prescription of +6.00 D with a vertex distance of 13mm.
Fcl = +6.00 / (1 - (0.013 x +6.00))
Fcl = +6.00 / (1 - 0.078)
Fcl = +6.00 / 0.922
Fcl = +6.51 D (rounded to nearest 0.25: +6.50 D)
The contact lens requires more plus power than the spectacle lens. Moving a plus lens closer to the eye decreases its effective power, so you need more actual power.
Understanding the Logic
Why does lens position matter? It relates to how a lens focuses light relative to the eye:
- A minus lens diverges light. When placed closer to the eye, the diverging rays have less distance to spread apart before entering the cornea, so the lens has a stronger effect. You therefore need less power when the lens is on the cornea
- A plus lens converges light. When placed closer to the eye, the converging rays have less distance to come together, so the focal point effectively moves farther back. You therefore need more power when the lens is on the cornea
Handling Prescriptions with Astigmatism
When converting a prescription that includes cylinder power (astigmatism), each meridian must be converted separately:
- Convert the sphere power using the formula
- Calculate the power in the other principal meridian (sphere + cylinder)
- Convert that combined power using the formula
- The new sphere is the converted value from step 1
- The new cylinder is the difference between the two converted values
For example, with a prescription of -6.00 -2.00 x 180:
- Convert -6.00 (sphere meridian at 180): Result = -5.58 D
- Convert -8.00 (power at 90, which is -6.00 + -2.00): Result = -7.30 D
- New prescription: -5.50 -1.75 x 180 (rounded to nearest 0.25)
Key Takeaways
- Vertex distance is the distance from spectacle lens to cornea, typically 12-14mm
- Conversion is needed when prescription exceeds +/-4.00 D
- Formula: Fcl = Fspec / (1 - d x Fspec), where d is in meters
- Minus prescriptions become less minus for contact lenses
- Plus prescriptions become more plus for contact lenses
- For prescriptions with astigmatism, convert each meridian separately
- Always convert vertex distance from mm to meters before calculating