- When a minus lens moves closer to the eye, it becomes stronger, and when it moves farther from the eye, it becomes weaker. Magnification and minification are also affected by vertex distance. - The vertex compensation formula is used to calculate power changes when a lens moves closer or farther from the eye. The power adjustment is equal to the lens power squared divided by 1000, multiplied by the distance of movement in millimeters. - The effective power of a lens varies with vertex distance and is calculated as the lens power divided by 1 minus the distance in meters multiplied by the power.

1. OPTOM.ASKAR.PK

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. • Example 1.
• Example 2

15. • 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:

17. • Example 1.
• Example 2

18. To be continue…………….

27. Specification of lens power
• Approximate power
• Back vertex power
• Front vertex power
• Effective power

28. Approximate power
• Fa = F1 +F2

30. • Fv = F1 /(1- t/n *F1 ) +F2

32. • Fv = F1+F2/(1- t/n *F2)

35. SPHERICAL EQUIVALENT