Vertex Distance and Calculations
R.D.Gopinath
Supervisor – Optical Sales
ARAVIND EYE CARE SYSTEM
Aravind Eye Hospital
& Postgraduate Institute of Ophthalmology
Madurai, India
Introduction
 Vertex distance is from the back surface of the
spectacle lens to the front surface of the eye.
 The vertex distance affects the effective power of the
lens, especially in higher powered (>4.0D)
prescriptions.
 The standard vertex distance is about 12mm.
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Lens Effectivity
 Lens optical effect may vary with vertex distance.
 Vertex distance responsible for decrease of vision.
 If moves away from eye,
+ lens becomes stronger.
-- lens becomes weaker.
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VD changes & the effect in ‘+’ lens
 Increasing the vertex distance of plus lens will
increase the effective power of the lens.
 Decreasing the vertex distance of a plus lens will
decrease the effective power of the lens.
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VD changes & the effect in ‘—’ lens
 Increasing the vertex distance of a minus lens will
decrease the effective power of the lens.
 Decreasing the vertex distance of a minus lens will
increase the effective power of the lens.
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Significance of VD
 In prescription must have the same effective power as
the refraction test.
 The vertex distance of the phoroptor / trial frame
must match the VD of the spectacle lenses.
 A vertex distance becomes significant if the diopter
power of the prescription exceeds 4.0D.
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Vertex compensation power
A spectacle is placed at 20mm away from the eye instead of 12mm, so
what is the actual prescription?
To determine this we need to use the vertex compensation formula:
Dc= Compensated Power
Dl= Original Lens Power
d= Change in Vertex Distance in Meters.
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Vertex compensation power
 A spectacle power of -5.00 D Sph 6mm further from
the eye than it should, then
>
The best sphere is – 4.85 D.
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Vertex Compensation Formula
 The formula for the needed compensation per mm.of displacement, per
diopter of lens power, is as follows:
= D x D / 1000
 The answer is multiplied by the mm of displacement. The result is added
or subtracted from the diopter power according to the following set of
conditions:

1) + lens moving closer - add to increase the diopter power

2) + lens moving farther away - subtract to reduce the diopter power

3) -- lens moving closer - subtract to reduce the diopter power

4) -- lens moving farther away - add to increase the diopter power.
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Example –spherical power lens
Consider a -12.00 Sph refracted at 13mm. The lens in the
patient's new glass will sit 10mm away from the patient's eye.
(12 squared = 144), 144/1000 = 0.14
The movement is 3mm closer to the patient's eye, with a minus lens.
3 x .14 = 0.42D,
so 0.5 D is subtracted from -12.00 to reduce the lens power to -11.50 D.
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Example – cylindrical power lens
 If the Rx has a significant cylinder power (at least 1 D), we must perform the calculation
for the primary meridians of power.
 A +12.00D S/ +3.00x180 lens, refracted at 14mm & Spec’s on pt’ face 10mm away.
 For +1200: 12 squared is 144, 144/1000 =0.14 mm.The movement is 4mm closer to the
patient's eye, with ‘+’ lens.
 4 x .14 =0 .48, so 0.5 D is added to +12.00 to increase the lens power to +12.50D.
 For +15.00: 15 squared is 225, 225/1000 = 0.225
 4 x .225 = 0.9, so 1 D is added to +15.00 to increase the lens power to +16.00.

Our adjusted Rx is: +12.50D S /+3.50 x 180*.
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How to measure VD?
 To determine VD use Distometer.
 Distometer
 places one arm on the eye lid while the other is
placed on the back of the lens, and
 a small scale attached to the device measures the
distance.
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