Building a Contact Lens Vertex Calculator: The Formula That Looks Simple Until It Isn't

The author built a contact lens vertex calculator and discovered hidden complexity beyond the simple vertex conversion formula (CL Power = F / (1 - d × F)). Key challenges included correctly handling cylinder (astigmatism) values—which must be derived by converting the combined sphere+cylinder meridian power rather than converting independently—and proper rounding to 0.25D increments, which must be applied only after all raw calculations are complete to avoid compounding errors.