What Is a Distometer?
A distometer is a specialized caliper-like instrument designed to measure vertex distance, the distance from the back surface of a spectacle lens to the front surface of the patient's cornea. This measurement is typically expressed in millimeters and ranges from about 10 to 15 mm in most fittings.
Why Vertex Distance Matters
The power of a spectacle lens at the eye depends on how far the lens sits from the cornea. For low prescriptions (under ±4.00 D), small changes in vertex distance have negligible effects. For high prescriptions, even a 2-3 mm change can significantly alter the effective power the patient experiences.
The relationship works as follows:
- Plus lenses moved closer to the eye: The effective power increases. The patient may be overcorrected.
- Plus lenses moved farther from the eye: The effective power decreases. The patient may be undercorrected.
- Minus lenses moved closer to the eye: The effective power decreases. The patient may be undercorrected.
- Minus lenses moved farther from the eye: The effective power increases. The patient may be overcorrected.
| Scenario | Effect on Plus Lens | Effect on Minus Lens |
|---|---|---|
| Lens moved closer | Power increases | Power decreases |
| Lens moved farther | Power decreases | Power increases |
Using the Distometer
The distometer has two ends:
- One end with a flat plate that rests against the closed eyelid (representing the corneal surface)
- The other end with a notch or step that contacts the back surface of the spectacle lens
To measure:
- Have the patient wear their glasses in the normal wearing position
- Ask the patient to close the eye being measured
- Place the flat plate gently against the closed eyelid
- Bring the lens contact end against the back surface of the lens
- Read the measurement in millimeters from the scale
- Repeat for the other eye if needed
Vertex Distance Compensation Formula
The formula to compensate for a change in vertex distance is:
F_new = F_old / (1 - d x F_old)
Where:
- F_new = the compensated power at the new vertex distance
- F_old = the original power at the original vertex distance
- d = the change in vertex distance in meters (positive if moving the lens closer, negative if farther)
For example, a -10.00 D lens refracted at 14mm vertex distance but fitted at 10mm (4mm closer, d = +0.004):
F_new = -10.00 / (1 - 0.004 x (-10.00)) = -10.00 / (1 + 0.04) = -10.00 / 1.04 = -9.62 D
The patient needs less minus power when the lens is closer to the eye.
When to Measure Vertex Distance
Measure vertex distance with a distometer in these situations:
- Prescriptions of ±4.00 D or higher
- When fitting frames that sit at a different distance than the trial frame or phoropter
- When converting between spectacle and contact lens prescriptions
- When a patient reports the new glasses feel different despite the same prescription
- When fitting unusually close (deeply curved) or far (flat) frames
Clinical Relevance
Vertex distance measurement is a quality control step that prevents patient complaints about lens power. For high prescriptions, a patient who says "my new glasses seem stronger (or weaker) than my old ones" may be experiencing a vertex distance difference rather than a prescription error. Measuring the vertex distance on both pairs can identify and resolve the issue without unnecessary remakes.
Key Takeaways
- The distometer measures the distance from the back lens surface to the front of the cornea
- Vertex distance affects effective power significantly for prescriptions ±4.00 D or greater
- Plus lenses gain effective power when moved closer; minus lenses lose effective power when moved closer
- Standard assumed vertex distance is approximately 13.75 mm
- The compensation formula is F_new = F_old / (1 - d x F_old)
- Always measure vertex distance for high prescriptions and contact lens conversions