Why is scleral lens power not calculated from the spectacle Rx?
Because a saline reservoir hundreds of microns deep sits in the light path, and its depth is not known until the lens is on the eye and has stopped moving.
Writing in Contact Lens Spectrum (September 2021), Ariel Cerenzie states the problem in exactly those terms: unlike corneal GP lenses, it is difficult to empirically estimate the required power of a scleral lens, because the base curve of the lens, the depth of the tear reservoir, the keratometry values and the relationship between those keratometry values and the back surface of the lens all interact — and the largest complication, compared with a corneal GP lens, is the thickness of the reservoir beneath the lens.
That breaks the chain of reasoning that works on a corneal GP lens. There, the tear layer between lens and cornea is thin, its power follows from radius against flat K, and the arithmetic is tractable enough to publish — it is on RGP Contact Lens Parameters and Starting Power. On a scleral lens the corresponding layer is thick, is deliberately not matched to corneal curvature, and shrinks over the wearing day. The spectacle refraction is still the input that gets you to a diagnostic lens of roughly the right power; it is not the number that gets ordered.
How much does settling actually move the power?
In theory about +0.25 D for every 70 µm of vault lost. In one measured study of normal corneas, the shift did not appear at all.
Cerenzie gives both halves. The theoretical figure comes from Munnerlyn’s formula: a 70 µm decrease in the reservoir should produce an over-refraction of approximately +0.25 D. Against that, Bray and colleagues measured an average of 83 µm of settling on normal corneas over six to eight hours and found no change in over-refraction — and concluded that although the vergence change over a normal eye did not produce a refractive shift, an over-refraction is still needed on irregular corneas.
Both facts are worth keeping. The theoretical relationship tells you which direction a power error will lie in when the vault has dropped more than you planned for, and roughly how large it can be: a lens that settles a couple of hundred microns further than expected is, on that relationship, a candidate for a shift of a few tenths of a dioptre in the plus direction, not several dioptres. The measured result tells you not to re-order power on arithmetic alone. On a regular cornea the predicted shift may be inside the noise; on an irregular cornea — which is most of this population — it must be measured rather than derived.
Why the reservoir has power at all
The saline sits between two surfaces with different curvatures — the back of the lens and the front of the cornea — so it forms a lens of its own, in the same way the thin tear layer under a corneal GP lens does. Change its thickness and you change the whole system. This is the same reasoning as the tear lens on a rigid corneal lens, scaled up by an order of magnitude and made unstable by settling.
How does an over-refraction become an ordered power?
By being taken through the settled lens and then compensated back to the plane the lens sits in before it is added to the lens power.
An over-refraction measured in a trial frame or phoropter is a spectacle-plane number, taken at roughly 12 mm in front of the eye. The lens it is being added to sits at the corneal plane. Below about ±4.00 D that difference rounds away at a 0.25 D step; above it, it does not, and a large over-refraction added straight onto a lens power is a real error rather than a rounding one. That compensation is what vertex-compensated contact lens power covers, and the vertex calculator performs it.
The residual arithmetic — combining a spherical or spherocylindrical over-refraction with the power already in the lens, and what remains when cylinder does not clear — belongs to the live Over-Refraction Calculator for Contact Lens Parameters. That page owns the over-refraction procedure on this site. Nothing on this URL duplicates it, and there is no widget here on purpose: a second over-refraction tool would be a worse copy of a page that already does the job.
Two practical conditions travel with the measurement. Take the over-refraction after the lens has settled, not at application — the settling section of Scleral Lens Vault gives the published timings. And take it through a lens whose fit you are willing to keep, because the next paragraph is the reason.
When do you re-order power, and when do you re-vault instead?
Change the fit first. A power ordered over a fit you are about to change is a number you will pay for twice.
The logic is mechanical rather than clinical judgement: if the sagittal height of the lens changes, the reservoir depth changes, and the power that was correct over the old reservoir is not correct over the new one. So any change that moves vault — a deeper or shallower sag, a different diameter, a landing-zone change that alters how far the lens settles — invalidates the power measured before it. The sequence those decisions belong to is on Scleral Lens Fitting Process.
| What you are seeing | What it usually is | What changes |
|---|---|---|
| Fit reads well after settling; a clean spherical over-refraction remains | A power problem | Power, once — over-refract and re-order |
| Vault has settled lower than planned; vision has drifted with it | A vault problem | Sagittal height first, then re-measure power |
| Vision fluctuates through the day or blurs before removal | Reservoir behaviour — debris, fogging, a moving fit | The fit and the reservoir, not the power |
| Acuity is not reached, and the over-refraction is spherocylindrical | Cylinder the lens is not correcting | A front-surface toric design, ordered with stabilisation |
| Landing zone blanches or the lens sits low, and vision follows the position | A landing and centration problem | Landing-zone geometry — power is downstream of it |
That last row is worth expanding, because it is the one most easily read as an optical problem. In “Landing in the Zone” (January/February 2024), Mile Brujic shows that a lens sitting inferiorly reads as unequal vertical clearance on OCT, and that the landing zone is one of the main causes — commonly a landing zone flatter than the trajectory of the sclera. A decentred optic zone in front of a pupil is a vision complaint that no power change fixes.
What sits with the laboratory rather than with an over-refraction?
Anything that changes the shape of the front surface rather than the sphere on the order form.
Cylinder that remains once sphere is optimised is ordered as front-surface toricity, and it needs the lens to hold its rotational position to be worth ordering — which is a landing-zone question before it is an optical one. Higher-order correction, decentred optics and wavefront-guided front surfaces are further along the same road: they are design decisions, made with a laboratory consultant against measurements of that eye. This site names them and does not attempt them. It publishes no laboratory nomogram, clones no manufacturer’s calculator, and lists no design by name.
What it does do is the plane arithmetic and the over-refraction arithmetic, on the two pages that own them: Contact Lens Vertex Calculator and Distance Chart and Over-Refraction Calculator for Contact Lens Parameters. Starting distance power from a spectacle Rx converts on the Contact Lens Conversion Calculator.
Starting parameters are not a prescription
A power derived on this site is a starting value for a trial lens, and on a scleral lens it is further from the final order than on any other design.
Read the site-wide bound on Starting Contact Lens Parameters Are Not a Prescription. For what settling does to the fit, read Scleral Lens Vault; for the sequence power sits at the end of, Scleral Lens Fitting Process; for the parameter set as a whole, Scleral Lens Parameters.
Sources
Clinical claims on this page are attributed to the publications below.
- Contact Lens Spectrum — Alright Folks, Let’s Settle Down(Ariel Cerenzie, OD; September 2021) — the difficulty of empirically estimating scleral lens power, with base curve, reservoir depth, keratometry and the keratometry-to-back-surface relationship all interacting and reservoir thickness as the largest complication; Munnerlyn’s formula giving approximately +0.25 D of over-refraction per 70 µm decrease; Bray et al measuring 83 µm of average settling on normal corneas over six to eight hours with no change in over-refraction, and concluding that over-refraction remains necessary on irregular corneas.
- Contact Lens Spectrum — Landing in the Zone(Mile Brujic, OD, FAAO; January/February 2024) — unequal vertical clearance on OCT as the read for an inferiorly positioned lens, with the landing zone a main cause, commonly a landing zone flatter than the scleral trajectory.
- Eef van der Worp — A Guide to Scleral Lens Fitting(Pacific University College of Optometry, open access) — sagittal height as the parameter that sets vault, and the dependence of clearance on lens sagittal height meeting the anterior ocular surface.
Constants used in the copy above — a 12 mm working vertex distance, an approximately ±4.00 D compensation threshold, and a 0.25 D ordering step — are the same values the calculators use, imported from the site’s conversion module rather than restated. How we calculate.
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