Vertex Distance
Effective power of a lens moved between two vertex distances. Spectacle plane → cornea uses 12 mm → 0 mm.
Inputs
Effective power
−7.30 D
Exact −7.2993 D at 0 mm
Nearest 0.25 D step
−7.25 D
From −8.00 D at 12 mm
High-power conversion: verify final lens power with manufacturer fitting guidelines and over-refraction.
Calculation
- 01Input: −8.00 D at 12 mm → 0 mm
- 02t = (12 − 0) / 1000 = 0.012 m
- 03F' = F / (1 − t · F)
- 04F' = −8.00 / (1 − 0.012 × −8.00)
- 05F' = −8.00 / 1.096
- 06F' = −7.2993 D → −7.25 D
Clinical verification required
Calculated result only. Confirm the final power by over-refraction over the trial lens and check the measured vertex distance of the actual frame.
Formula
- 01F' = F / (1 − t · F)
- 02t = (original vertex − new vertex) in metres
- 03F = original power in dioptres
Clinical application
Limitations
Vertex correction becomes clinically relevant above roughly ±4.00 D and is essential above ±7.00 D. It assumes an accurately measured vertex distance and a thin-lens model.
OptiX Pro provides mathematical calculations, reference information, and fitting assistance. It does not replace professional clinical examination, manufacturer fitting guidelines, or professional judgement. Contact lens selection and prescription must be clinically verified.