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

I needed a contact lens vertex calculator for a tool site I'm building. Seemed straightforward — there's a known formula, plug in the numbers, done. It wasn't done. Here's what I actually ran into. What Vertex Distance Is (Quick Background) When you wear glasses, the lens sits about 12mm in front of your eye. A contact lens sits directly on the cornea — effectively zero distance. This matters because optical power isn't linear. The same lens power behaves differently depending on how far it is from the eye. A -6.00D spectacle lens doesn't deliver -6.00D to the corneal plane. It delivers slightly less. The formula that corrects for this is called the vertex conversion formula: CL Power = F / (1 - d × F) Where: F = spectacle lens power in diopters d = vertex distance in meters (so 12mm = 0.012) Plug in -6.00 at 12mm: CL = -6.00 / (1 - 0.012 × -6.00) CL = -6.00 / (1 + 0.072) CL = -6.00 / 1.072 CL ≈ -5.597 Rounded to the nearest 0.25D (how contact lenses are actually manufactured): -5.75D.