Work out contact lens vertex conversion instantly with clear inputs, formula shown and shareable results.
Applies the vertex distance formula to convert a spectacle prescription to the corneal plane, which matters above about 4 dioptres.
Vertex conversion
Fc = Fs / (1 - d x Fs), where d is the vertex distance in metres and Fs the spectacle power in dioptres
Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.
Above roughly 4 dioptres, where the difference exceeds a quarter dioptre step.
Minus lenses become weaker at the cornea and plus lenses become stronger.