Which pages cover contact lens parameters?
Four pages cover the ordered set: converted power, base curve, diameter, and the RGP starting values.
The table lists each page, the value it covers, and when to open it. Convert power first, then fit BC and diameter with the ordered sphere. Take the RGP guide when the path runs from keratometry to a GP base curve and power.
| Page | What it covers | When to open it |
|---|---|---|
| Contact Lens Conversion Calculator | Starting ordered sphere at the corneal plane aftervertex-compensated contact lens power | Glasses Rx in hand; convert power first |
| Base Curve of Contact Lenses | The radius you order with converted power | After conversion; fit BC on eye |
| Diameter of Contact Lenses | The overall size you order with converted power | Fit diameter on eye with base curve |
| 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 |
Soft base curve and diameter stay on those two explainers. Definitions live there, not on this inventory. GP keratometry-to-radius belongs on the RGP guide.
Why is the ordered set three values, not one?
A written glasses refraction supplies exactly one of the three: power, and only after the plane change is applied. The other two describe geometry the refraction never measured. Base curve is the radius of the back surface that has to land on a specific cornea; diameter is how much of that cornea the lens covers. Neither number exists anywhere on a spectacle Rx, which is why no calculator on this site will invent them.
The two geometric values also move together. A steeper base curve or a larger diameter each deepen the lens vault, so fitters read them as a pair against how the lens centers, moves, and settles — behavior only visible at the slit lamp. That is the practical reason the inventory splits the way it does: the converted value can be computed here, the fitted pair cannot.
When a printed box lists BC and DIA next to power, those two came from the manufacturer's available geometry and the fitter's judgment, not from arithmetic on the refraction.
How do power, base curve, and diameter connect?
Power is converted. Base curve and diameter are fitted with that ordered sphere. The Contact Lens Parameter Guides define related optics; this inventory lists the ordered set.
- Convert spectacle prescription to contact lens parameters on the Contact Lens Conversion Calculator.
- Fit Base Curve of Contact Lenses on eye with the ordered sphere.
- Fit Diameter of Contact Lenses on eye with base curve.
- Open RGP Contact Lens Parameters and Starting Power when the lens is GP, not soft.
The bound that applies across this site is written up in Starting Contact Lens Parameters Are Not a Prescription. Convert power first. Fit BC and diameter. Verify on eye. Everything before the trial lens is a starting value.