What is vertex distance in eyeglasses?

Vertex distance is the distance from the back surface of the spectacle lens to the front surface of the cornea, measured in millimeters. It typically ranges from 10 to 15 mm. This distance affects the effective power of the lens at the eye and is clinically significant for prescriptions of ±4.00 D or greater.

When does vertex distance matter?

Vertex distance becomes clinically significant for spectacle prescriptions of ±4.00 D or greater. Below this threshold, small changes in vertex distance produce negligible power changes. It also matters when converting between spectacle and contact lens prescriptions, since contact lenses sit directly on the cornea (zero vertex distance).

What happens if vertex distance is wrong?

If the glasses are worn at a vertex distance that differs significantly from the refraction distance, the patient experiences a different effective power than prescribed. This can cause blur, over-correction, or under-correction. For example, a -10.00 D lens worn 4mm closer than refracted would feel about 0.40 D weaker, which may be noticeable to the patient.

How do you use a distometer?

Place the flat plate end against the patient's closed eyelid and bring the notched end against the back surface of their spectacle lens while they wear the glasses in normal position. Read the measurement in millimeters from the instrument's scale. Measure each eye separately if the frame sits asymmetrically.

Distometer: Measuring Vertex Distance for Accurate Spectacle Prescriptions

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

Vertex distance compensation is clinically significant for prescriptions of ±4.00 D or greater. Below this threshold, the power changes are small enough that most patients do not notice. The ABO exam frequently tests the ±4.00 D threshold as the point where vertex distance becomes important.

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

The standard assumed vertex distance when it is not specifically measured is 13.75 mm (sometimes rounded to 14 mm). If the refraction was performed at this standard distance but the glasses are fitted at a significantly different distance, a vertex distance compensation should be applied to the prescription for powers above ±4.00 D.

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.

Ignoring vertex distance when converting between spectacle and contact lens prescriptions. Contact lenses sit directly on the cornea (vertex distance = 0), so high spectacle prescriptions must be compensated. A -10.00 D spectacle Rx becomes approximately -9.25 to -9.50 D in a contact lens, depending on the original vertex distance.

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