Vertex distance

OPTOMETRY

1. OPTOM.ASKAR.PK Vertex distance and power

2. 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

3. 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

4. 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

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

6. 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

7. Example 1. Example 2

8. 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:

9. Example 1. Example 2

10. To be continue…………….

11. Specification of lens power Approximate power Back vertex power Front vertex power Effective power

12. Approximate power Fa = F1 +F2

13. Fv = F1 /(1- t/n *F1 ) +F2

14. Fv = F1+F2/( 1- t/n * F2)

15. SPHERICAL EQUIVALENT

16. Thank you