This document discusses different ways to specify the power of a lens, including approximate power, back vertex power, front vertex power, and effective power. It also provides formulas for calculating front vertex power and spherical equivalent power based on the lens's focal length, thickness, and refractive index.

10. SPECIFICATION OF LENS POWER
• Approximate power
• Back vertex power
• Front vertex power
• Effective power

11. APPROXIMATE POWER
Fa = F1 +F2

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

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

18. SPHERICAL EQUIVALENT