Prescription Conversions in Clinical Practice
Spectacle to Contact Lens Power (SRx to CLRx)
Spectacle lenses sit about 12 to 14 mm from the cornea. When that distance goes to zero with a contact lens, effective power changes. Minus powers lose effect, plus powers gain. Adjust any meridian at or beyond ±4.00 D, and always beyond ±6.00 D, to avoid residual blur, adaptation problems, and unnecessary refits. Normalize the refraction (minus cylinder), identify high power meridians, apply vertex, round to available steps, then confirm with a diagnostic lens and over refraction.
Vertex Distance Math and When It Matters
Use the effective power formula: F_c = F / (1 - dF) (F in diopters, d in meters). Calculate per principal meridian when cylinder is present, then transpose if needed. Document the assumed vertex for every high power CLRx so future changes make sense. Understanding the math improves counseling, speeds troubleshooting, and keeps you in control even when software automates the step.
Cylinder Decisions: Spherical Equivalent or Toric?
Spherical equivalent (SE) = Sphere + (Cylinder ÷ 2). It can be acceptable for low regular astigmatism (often ≤0.75 D) when axis repeatability is poor, cost is critical, or the patient is asymptomatic. Avoid SE in night drivers, higher cylinder powers, or anyone who reports ghosting. If diagnostic over refraction shows meaningful residual cylinder, move to toric. Better acuity beats theoretical simplicity.
Plus/Minus Cylinder Transpose and Power Checks
Fast transpose refresher: add sphere and cylinder to get the new sphere, change cylinder sign, rotate axis 90 degrees. Example: +2.00-1.50x180 → +0.50+1.50x090. Always recheck powers before and after vertexing so rounding happens only once. This prevents compounding errors when you jump between refraction notation, calculators, and ordering systems.
RGP Tear Lens Power (SAM, FAP) and Ordered Power
With rigid lenses, the tear (lacrimal) lens adds plus when you fit steeper than K (SAM: steep add minus to ordered power) and minus when flatter (FAP: flat add plus). Ordered power = spherical over refraction ± tear lens contribution, vertexed if high. Workflow: capture Ks or topography, pick base curve, evaluate fluorescein pattern, measure over refraction, compute final power, and decide on front surface toric or bitoric if residual cylinder persists.
Radius/Diopters, Over Refraction, and Documentation
Convert base curve radius to diopters with D = 337.5 / r(mm) to sanity check fits or explain changes. During CL over refraction, keep powers in minus cylinder and vertex only once at the end. Record every assumption: vertex used, rounding step, chosen design, expected residual cylinder. Thorough notes speed future refits, make staff training easier, and reduce remake costs.