Tolerating vertex distance changes for spherocylindrical corrections.

Prescriptions depend on the vertex distance. Although weak prescriptions are insensitive to a vertex distance change, stronger ones must be adjusted if the vertex distance is modified by an appreciable amount. The tolerable amount can be specified for pure spherical powers. For spherocylindrical corrections, the concept of dioptric distance is invoked in this article and applied to the propagation of an astigmatic wavefront. This leads to a simple rule that is well suited to decide on the necessity of modifying a prescription.