How do we predict the lens power needed when fitting a GP lens empirically?

• A. Use keratometry alone
• B. Vertex converted SRx - tear layer power
• C. Trial lenses with no tear layer consideration
• D. Use a fixed power for all eyes

Answer: (B)

Predicting GP lens power empirically comes from translating the patient’s refraction into the corneal plane and accounting for the tear layer between the cornea and the lens. Start with the spectacle prescription and convert it to the corneal plane using vertex distance (vertex conversion). Then subtract the tear layer power, which is the refractive effect of the tear film that sits between the cornea and the posterior surface of the GP lens. This tear layer acts as a small corrective layer, so ignoring it would give an inaccurate lens power. Keratometry alone misses the patient’s refractive error and the tear-layer influence, trial lenses without considering the tear layer miss a key factor that shifts the effective power, and a fixed power for all eyes fails to reflect individual differences in refraction and corneal shape.