What differs in the ordered parameter set?
Almost everything. The two families share a purpose and share no parameter names except diameter.
| What you specify | Soft centre-distance design | Orthokeratology design |
|---|---|---|
| Primary input | The refraction | The topography — central radii, eccentricity and HVID |
| Distance power | Ordered, as a vertex-compensated sphere at the corneal plane | Not ordered as a power; expressed as the target flattening built into the back optic zone radius |
| Treatment optic | A stated add, selected from what the design offers | The reshaped corneal profile itself; no add is ordered |
| Back surface | One base curve, usually from a short list per design | Four or five curves: back optic zone, reverse, alignment, peripheral |
| What sets the vault | Base curve and diameter together | Reverse curve depth and alignment zone radius or angle |
| Diameter | Usually fixed per design | Ordered, and sized against HVID |
| Verification | Over-refraction and centration at the slit lamp | Post-wear topography difference map, read before the refraction |
| When it is worn | Daytime | Overnight; nothing is worn during the day |
The row worth dwelling on is what sets the vault. On a soft lens, base curve and diameter act together as the pair described on Sagittal Depth of Contact Lenses. On a reverse-geometry lens the base curve has already been spent on producing central flattening, so, as Mountford and Noack (Contact Lens Spectrum, June 2002) set out, sagittal height is controlled instead by the reverse curve depth and the alignment zone. Two lenses, two entirely different levers for the same geometric quantity.
When is a converted spectacle power the starting point, and when is it not?
On the soft side it always is. On the reshaping side it never is.
A soft centre-distance design still needs a distance zone that gives clear vision, and that zone carries the same vertex-compensated sphere any soft lens would — calculate it on the Contact Lens Conversion Calculator and read vertex-compensated contact lens power for why the spectacle and corneal planes diverge above about ±4.00 D. The treatment optic sits around that number; it does not substitute for it.
An ortho-k lens has no power zone in the ordinary sense. The amount of myopia to be reduced enters the design as flattening applied to the back optic zone radius relative to flat K — the Jessen and compression factor arithmetic on RGP Contact Lens Parameters and Starting Power. Converting a spectacle sphere and ordering it as a power would be a category error: there is nothing on the lens for it to be ordered into.
The verification step inherits that split. A soft fit is verified with over-refraction through the trial lens. A reshaping fit is verified with a post-wear topography difference map, and Mountford and Noack are explicit that the map is taken before the refraction, because a refraction measured over a decentred treatment zone carries no usable information. Details of that reading are on Ortho-K Lens Design and Post-Wear Topography.
What are the toric limits in each family?
Both families run out of astigmatic range, and they run out for different reasons — one optical, one mechanical.
Most soft myopia control designs carry spherical distance powers only. Melanie Frogozo, OD (Review of Cornea & Contact Lenses, February 2026) describes the practical consequence: where a significant cylinder does not correct acceptably in the chosen design, consider a design that offers astigmatic correction, and where none is available, spectacles worn over the lenses for the residual error are an option. The limit here is that the cylinder cannot be ordered, not that the eye cannot be corrected.
In a reshaping design the limit is that the lens must land 360° on the mid-periphery. Astigmatism is handled with a toric landing zone rather than a cylinder power. Myopia Profile gives around 1.50 D of corneal toricity, or a sagittal height difference of around 30 µm over an 8 mm chord, as generally agreed thresholds — noting that the chord and difference vary between manufacturers.
Neither of those is soft toric rotation, which is a third problem again. Starting sphere, cylinder and axis, the stability assessment and the LARS compensation live on the Toric Contact Lens Calculator.
Which page covers what?
This page is the parameter comparison. The optics and the geometry each have their own page, and the calculators stay where they are.
- Myopia Control Contact Lenses: Optics and Ordered Parameters — the category, and what the two families have in common.
- Myopic Defocus and Treatment Add of Myopia Control Lenses — what the plus power outside the distance zone is doing, and the zone geometries that deliver it.
- Ortho-K Lens Design and Post-Wear Topography — the reverse-geometry curves and how the post-wear map verifies them.
- Multifocal Contact Lens Calculator — the presbyopic starting add, which is a different procedure from a treatment add.
This site publishes no brand parameter grids and no comparison of one manufacturer's design against another's. What it can compare is what each design family asks you to write down, which is the comparison above. Which family a patient should be in, at what age, and on what schedule are clinical decisions this site does not make.
Read Starting Contact Lens Parameters Are Not a Prescription for the bound that applies across this site.
Sources
Clinical claims on this page are attributed to the publications below.
- Melanie Frogozo, OD, Succeed with Soft Multifocals (Review of Cornea & Contact Lenses, February 2026) — myopia control soft designs carrying spherical distance powers; considering a design offering astigmatic correction where cylinder does not correct acceptably, and over-spectacles for residual error where none is available; lathe-cut designs allowing specified base curves, diameters and zone sizes.
- John Mountford and Don Noack, Corneal Topography and Orthokeratology: Post-fit Assessment (Contact Lens Spectrum, June 2002) — reverse-geometry sagittal height controlled by the reverse curve depth and the alignment zone radius or angle rather than by base curve and optic zone diameter; the post-wear map taken before the refraction.
- Myopia Profile, What topography data do I need to fit orthokeratology lenses?— sagittal height from central radii and eccentricity over a stated chord; lens diameter sized against HVID; around 1.50 D of corneal toricity, or around 30 µm of sagittal height difference over an 8 mm chord, as generally agreed toric-design thresholds that vary between manufacturers.
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