This document discusses vertex compensation and how the effective power of a lens changes with vertex distance. It provides the vertex compensation formula of D2/1000 to calculate the amount of power added or subtracted when a lens moves closer or farther from the eye. An example shows a +12.00D lens moving 4mm closer, requiring an addition of 0.50D. The effective power formula is also provided to determine contact lens power from spectacle power.

5. In case of minus lens ,become weaker while it moving
from the eye and become stronger while moving
closure to the eye
• Magnification & minification can affected
significantly by the VD
• Greater the VD, greater the degree of Magnification
& minification
• Image size changes are minimized when VD is zero

8. VERTEX COMPENSATION FORMULA
Compensation /mm of displacement per diopter of lens power,
is
D²/ 1000
The result is added or substracted from the power according to
following set of condition:
 Add when
• Plus lens moving closer
• Minus lens moving farther
 Substract when
 Plus lens moving farther
 Minus lens moving closer

9. EXAMPLE 1
For +12.00 D the movement is 4mm closer to
the patient
D²/1000
12²/1000
144/1000 = 0 . 14
4 x0 . 14 = 0 . 48 (0 . 50 aprox)
0.50 is added to +12.00

10. Note: if the prescription has a
significant cylinder power (at least
1D) we must perform the calculation
for the primary meridians of power

13. EFFECTIVE POWER
The effective power of a corrective
lens varies with the vertex
distance and the formula is
Fe = F/ 1- dF
d → distance in meter

14. If the power of the required spectacle is known,
the power of CL required to correct the same
eye can be determined by means of the
effective power formula:

16. TO BE CONTINUE…………….

17. THANK YOU….