How does lens-induced astigmatism occur? Have you always wondered? How is it that you start out with no astigmatism, and then after you start wearing a small amount of correction, it increases after a few years just like your myopia? And then a few years later, it increases again along with your myopia. And so-on, until you’re wearing several diopters of cylinder. How?
It’s pretty simple.
Lens-induced astigmatism tends to occur under the following conditions:
- You’re given a small initial cylinder power, like 0.50D
- Your spherical powers are later overcorrected.
A quote right from the source that sums it up nicely:
“For every half diopter the patient is over-minused, the cylinder plus power needs to be increased by one diopter to maintain the spherical equivalent and keep the circle of least confusion on the retina.” (p.55, “Subjective Refraction and Prescribing Glasses”, Kolker, Richard J. MD.) Link: https://www.aao.org/Assets/563fc40b-1466-477e-bc12-4e62f8b2d324/635476894936870000/subjective-refraction-prescribing-glasses-pdf
In other words, remember that pesky little spherical equivalent formula that we use to convert cyl to sph? That is what is responsible for the relationship between the two that causes your cyl to get worse over time. So, if someone is overcorrected by 1 diopter in spherical, their cylinder could end up overcorrected by two diopters.
So there you have it! That is how lens-induced astigmatism begins, quoted directly from a well-respected refraction manual by an ophthalmologist.