Calculation of spectacle lens optical power if vertex distance is changed
Introduction
Change of vertex distance of the spectacle frame influences lens vertex power (SB). The most often we meet this problem in contact lens application. In this case is difference about 12 mm in vertex distance. Next example is recalculation of the new vertex power from trial frame to target frame. In this case the change is from 3 to 6 mm in vertex distance.
Goals
- Measure vertex distance in trial frame (dZ)
- Measure vertex power of the trial frame (SBZ)
- Measure vertex distance of the individual spectacle frame (dI)
- Calculate vertex power for individual lens (SBI)
Equipment
Spectacle frame, spectacle lens, ruler, millimeter ruler, writing equipment, calculator, angle meter.
Methods
Measure vertex distance in trial frame (dZ)
With help of handy PD meter measure vertex distance – distance between back side of the lens and front side of the cornea.
Measure vertex power of the trial frame (SBZ)
Measure vertex power of the lens in lensmeter.
Measure vertex distance of the individual spectacle frame (dI)
With help of handy PD meter measure vertex distance – distance between back side of the lens and front side of the cornea.
Calculate vertex power for individual lens (SBI)
Use the below listed formula to calculate vertex distance of the individual adapted lens.
Results
Measure vertex distance in trial frame (dZ)
dZ =
Measure vertex power of the trial frame (SBZ)
S´BZ =
Measure vertex distance of the individual spectacle frame (dI)
dI =
Calculate vertex power for individual lens (SBI)
S´BI=
Discussion
Change of vertical power should be done above 4 D. Below this power the change is smaller than 0.25 D (in case of standard vertex distance 12 mm). In case of usage of concave spectacle lens stronger more than 4 D we decrease the power of the applied contact lens.
Conclusion, notes, comments
In the trial frame we have spectacle lens -3,5 D. We will apply contact lens. What vertex power will contact lens have?