How To Convert A Glasses Prescription Into Contact Lenses
Why Contact Lens Conversions Matter
Contact lenses sit on the cornea, while spectacles are about 12 to 14 mm away. Minus powers weaken and plus powers strengthen at the corneal plane. Without a vertex adjustment, higher prescriptions (typically ≥ ±4.00 D and always ≥ ±6.00 D) can leave patients with blur, eyestrain, and headaches. Precise conversion protects acuity, comfort, and confidence in your fit.
Applying the Spherical Equivalent
Spherical equivalent (SE = sphere + cylinder ÷ 2) is useful for mild regular astigmatism (about 0.75 D or less) when a toric lens offers little practical gain or cost is a concern. Choose a toric design when residual cylinder affects night driving, detailed work, or shows clearly on diagnostic over refraction.
Vertex Distance: Simple Formula and Steps
Use Fc = F / (1 - dF) where F is the power in diopters and d is vertex distance in meters. Convert to minus cylinder, identify meridians at or beyond the vertex threshold, apply the formula to those meridians, round to 0.25 D steps, then verify with an over refraction before finalizing the CLRx.
Key Clinical Considerations
Math is only part of the fit. Corneal curvature, tear film stability, material choice (SiHy), lens type (sphere, toric, multifocal), and modality selection all influence success. Topography and keratometry help predict residual astigmatism and guide design selection so patients achieve clear, comfortable, long term wear.