Which guides define contact lens parameters?
Two guides anchor the library: one on vertex distance, and one on RGP parameters and starting power. Both are definitions with worked optics, not tools.
The table lists each guide, the value it defines, and when to open it. Start with vertex distance. Open the RGP guide when the path runs from keratometry to a GP base curve and power.
| Guide | What it defines | When to open it |
|---|---|---|
| Vertex Distance of Contact Lenses | Why glasses power is not contact lens power | You need the optics before you calculate on the vertex tool |
| RGP Contact Lens Parameters and Starting Power | Starting RGP parameters from keratometry and spectacle power | GP path; calculate BC and power, then verify with over-refraction |
The vertex guide is the why; the Contact Lens Vertex Calculator and Distance Chart is the tool. The RGP guide is the GP node. Soft base curve and diameter live on the parameter pages instead.
How should you read a guide before opening a calculator?
Read for the bound first. Every definition in this library ends at the same wall: a number computed from a written refraction is a starting value, and the eye gets the final vote. Knowing where that wall sits tells you how much to trust the arithmetic — fully, up to the trial lens; not at all past it.
Then read for the units. Vertex distance is millimeters between two planes; get it wrong by three and a −9.00 D myope shifts a quarter diopter. Keratometry arrives as either millimeters of radius or diopters of curvature, and the RGP guide exists partly because D = 337.5 ÷ r trips people in both directions. The guides spell out which unit each formula wants so the calculators can stay strict about what they accept.
Skip ahead when the case is routine. A −2.00 D sphere needs no vertex theory — compensation at that power is smaller than a manufacturing step. The guides earn their reading time at high powers, mixed meridians, and GP fits, which is exactly where a memorized rule of thumb starts to fail.
How do these guides connect to calculators and ordered parameters?
These guides define parameters. The calculators convert and calculate starting power. Ordered base curve and diameter are fitted, not produced by a glasses Rx.
- Calculate vertex-compensated power on the Contact Lens Vertex Calculator and Distance Chart after the vertex-distance guide.
- Fit and verify on eye. Starting parameters are not a prescription — see Starting Contact Lens Parameters Are Not a Prescription.
- Order soft Base Curve of Contact Lenses with converted power. GP keratometry-to-radius belongs on the RGP guide.
- Order Diameter of Contact Lenses with converted power and fitted base curve.
The order of operations does not change: define the value, convert it, fit the trial lens, verify with over-refraction. Everything computed before the trial lens stays a starting value until a fitter checks it on eye.