vertex distance with a free form lens

[eyeoneder]

how important is it to include the vertex distance when ordering a free form lens?

[Barry Santini]

DEpends. Only specify Vertex where you know it departs in fitting from the refractive vertex...if known, IMHO.

B

[Darryl Meister]

In a fully customized free-form lens, the fitted vertex distance is used to calculate the angles of view at different points over the lens design during the optical optimization process. This allows the software to calculate the correct optics at each point over the lens as the software ray traces the lens design for the position of wear. If your free-form lens supplier only requires one back vertex distance, there is a good possibility that the supplier is referring to the fitted vertex distance for use in these optical ray-tracing computations.

This is not the same as compensating the prescription for vertex distance, which only involves modifying the powers slightly using a simple equation in order to account for the difference in vertex distance between the fitted and refracted lenses. Compensating the prescription for the vertex distance typically requires both a fitted vertex and a refracted vertex distance. This type of compensation can easily be performed by the eyecare professional. When in doubt, you should ask your free-form lens supplier for clarification.

Best regards,
Darryl