Which pages cover contact lens parameters?
Fourteen pages cover the ordered set: converted power, base curve, diameter, sagittal depth, thickness, oxygen transmissibility, the material those values are made in, the printed fields they are written in, which eye each set belongs to, 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 |
| Sagittal Depth of Contact Lenses | The vault height base curve and diameter produce together, over a stated chord | Base curve alone does not explain how the lens sits |
| Center and Edge Thickness of Contact Lenses | The geometry the ordered power sets, quoted in millimeters at the lens center | You need to read a printed thickness value or an edge profile |
| Oxygen Transmissibility and Dk/t of Contact Lenses | What that thickness does to corneal oxygen, plus the material composition behind Dk — water content, hydrogel versus silicone hydrogel, and modulus | You compare materials, or ordered power moved the divisor |
| What Contact Lenses Are Made Of | The polymer families, and what the material decides about the rest of the set | Before base curve and diameter — the material constrains both |
| How to Read a Contact Lens Prescription | Every printed field on the label, and who set it | A document is in front of you and the abbreviations need decoding |
| Is Contact Lens Power the Same as Sphere? | The power field, and why a spectacle sphere is not the ordered one | Before converting; it is the field the conversion acts on |
| What Does CYL Mean on a Contact Lens Box? | The astigmatic power a toric lens carries, in diopters | A toric order is on the table and CYL needs reading |
| What Does Axis Mean for Contact Lenses? | The meridian, 001–180, that the cylinder is placed along | Same order; axis is the one field conversion never changes |
| Low, Medium, and High Add of Multifocal Contact Lenses | What a LOW, MED or HIGH add designation is, and is not | A multifocal is being ordered and the band has to be chosen |
| OD vs OS for Contact Lenses: Which Eye Is Which | Which eye each parameter set belongs to, right or left | Before either eye is converted, so the two sets are not swapped |
| 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.
Two questions arrive before the table is useful, and both are routed below: which fields the set is written in, and which material it is made in.
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.
Which fields is the ordered set written in?
Power and sphere are one field on a contact lens, and the spectacle sphere is not that number until it is vertex-compensated — the full answer is on Is Contact Lens Power the Same as Sphere?. The printed document those fields appear on is read end to end on How to Read a Contact Lens Prescription.
The ordered set enumerates as four groups, and each has its own page:
- SPH / PWR / PWR-D — one sphere power per eye. Convert it on the Contact Lens Conversion Calculator; the field itself is on Is Contact Lens Power the Same as Sphere?.
- CYL and AX — only on a toric lens. Read What Does CYL Mean on a Contact Lens Box? and What Does Axis Mean for Contact Lenses?, then calculate starting SPH, CYL, and AX on the Toric Contact Lens Calculator.
- ADD — only on a multifocal, and usually a designation rather than a dioptric value: Low, Medium, and High Add of Multifocal Contact Lenses. Starting add belongs on the Multifocal Contact Lens Calculator.
- BC and DIA — geometry, not power. They are fitted, never read off the refraction.
Whether the sphere written for glasses can be ordered as the sphere for contacts is a different question, and it is answered on Starting Contact Lens Parameters Are Not a Prescription. Vertex-compensated contact lens power is what moves a spectacle sphere to the corneal plane.
Which material is the ordered set made in?
One of three polymer families — hydrogel, silicone hydrogel, or rigid gas permeable plastic — and the family decides which base curves and diameters exist to be ordered at all, which is why material sits in this inventory. The composition itself is answered on What Contact Lenses Are Made Of, and what a material transmits on Oxygen Transmissibility and Dk/t of Contact Lenses.
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.
- Read Sagittal Depth of Contact Lenses for what those two produce together over a stated chord.
- Read Center and Edge Thickness of Contact Lenses for the geometry the ordered power sets, thenOxygen Transmissibility and Dk/t of Contact Lenses for what that thickness costs the cornea.
- 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.