What happens during an over-refraction
You wear your scleral lens and read an eye chart while your fitter places lenses in front of your eye, either in a trial frame or a phoropter, and asks which looks clearer. The result is the extra correction you still need on top of your lens. Your fitter then orders a new lens with that correction built in, so the lens corrects as much as possible on its own.
It’s the same idea as a regular glasses test, just done over the lens. That’s where the name comes from.
Why a scleral lens needs one
With a scleral lens, your vision is shaped by two things together: the lens itself and the layer of saline between the lens and your cornea. The saline fills in the irregular front of your cornea, which is why scleral lenses help so much with conditions like keratoconus. But it also means changes in the fit can change your prescription.
- Vault depth. In a pilot study of 15 healthy eyes, fitting lenses with progressively deeper vaults shifted the prescription toward nearsightedness and increased some optical distortions.[3]
- Flexing. A lens that doesn’t sit evenly on the white of the eye can bend slightly, adding unwanted astigmatism. In a small study, a toric landing zone reduced that flexure by about 0.37 diopters on average compared with a spherical landing zone.[4]
- Optical type. In an international survey, the optics in scleral lenses were spherical in 70% of lenses, toric in 27%, and designed to correct higher-order aberrations in 3%.[5]
Why timing matters
A scleral lens sinks slightly into the tissue on the white of the eye during the first hours of wear. In 22 keratoconus eyes, about 80% of that settling happened within the first 4 hours.[6] In 15 healthy eyes, settling and lens decentering mostly plateaued within 2 to 4 hours, and the change in the saline layer altered its optical power by only about 0.12 diopters.[2]
What does that mean for your prescription? In a study of 16 people with normal corneas, the over-refraction was measured 30 minutes after insertion and again after 6 to 8 hours. The saline layer thinned by an average of 83 microns, but the sphere and cylinder in the over-refraction changed very little, and vision didn’t change significantly.[1] Those results were in normal corneas; the authors didn’t test irregular corneas, where changes may differ.
In practice, many fitters re-check the over-refraction after the lens has settled before finalizing your prescription, especially if your vision seems to change over the day.
What you can do
- Give honest answers. “They look the same” is a useful answer. Don’t guess.
- Describe what you see in daily life. Mention if your vision is sharp in the morning but softer later, or if one eye seems different.
- Rule out the simple causes first. If your vision is blurry right after insertion, it’s often a bubble or debris rather than the prescription. See blurry after insertion.
If your vision is still not where you’d like after the prescription has been fine-tuned, see vision still not perfect with sclerals?
Common questions
Should I wear my lenses to my follow-up appointment?
Usually, yes, if your fitter asks you to. Arriving with lenses that have been on for a few hours lets your fitter see them settled and check your vision in the state you'll actually be wearing them. Bring your case and solutions in case they need to take the lenses out. Follow your own fitter's instructions.
Why does my vision seem sharper in the exam room than at home?
Exam rooms have controlled lighting and high-contrast charts. Real life includes dim light, screens, and glare. Tell your fitter where and when your vision falls short, such as night driving or reading, so they can take that into account.
Why do I still need reading glasses over my scleral lenses?
The over-refraction usually sets your distance vision. As people get older, focusing up close naturally gets harder, so you may need reading glasses or a lens designed for near and far. Ask your fitter about your options.