This is how lens-induced astigmatism occurs

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:

  1. You’re given a small initial cylinder power, like 0.50D
  2. 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:

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.


Seldom do things go exactly as predicted by a formula. And changes in notation make discerning what is happening more convoluted. Here is a real-life example:

Let’s say you start with:

-2.00 SPH

You get a new exam two years later, and are prescribed:

-2.50 SPH -0.50 CYL

The following year, at a new optometrist you’re prescribed (note the change in sign of the cyl from one office to the next as we proceed):

-3.50 SPH +1.50 CYL, exactly as predicted by the formula

Three years later, you go to a new optometrist who sees your vision has worsened, but who plays it conservative by not increasing your SPH or CYL as much as the formula suggests, and thinks he’s really, really helping you out by that:

-4.00 SPH +1.50 CYL

Fifteen years later, while pregnant, you go to another new optometrist, who seems to reduce the sphere slightly (and says it’s hormones causing the change), but also adds in some of the missing cylinder from the previous exam (because you’re still overcorrected, and now your eye/brain system is starting to expect a good amount of cyl during subjective refraction):

-3.50 SPH -2.25 CYL

But notice that’s minus cylinder again, because that’s the notation and equipment that particular office uses. Sneaky! Your sphere has actually increased a lot more than you think, along with your cylinder, due to the increase being masked by the change in notation again. You actually went up in both sphere and minus cylinder this time!

Uh oh!


Awesome topic, well explained. Love the source. Can I quote in the blog?


@jakey, Let me change my profile pic first, please, in case you take a screen shot. I will try to do it in a bit. Thank you for asking first.



I’m flattered.

Profile pic is now changed to kitteh! If you could also blur out my username in any screen shots you take, that would be appreciated. Thanks.

Are you just using stuff from my initial post in this thread?


That’s what I was thinking. And sure, I’ve got all teh blurs set to maximum!


So this is how i ended up with -5 SPH -3 CYL
Free eye exams if you buy glasses of course…

Any insights on what is happening to the eye when you wear overcorrected astigmatism?
To the best of my knowledge and experience, the cilliary muscle can only correct spherical. (Am i wrong?)
So the eye must have some mechanism of becoming more egg-like?
Or, your eye starts up as an egg but before you start wearing correction, the cylinder is negligible, and as the eye (egg :egg:) grows it grows with it.

Thank you for sharing :hugs:


@itamar, my guess would be like you said, as the eye becomes more myopic, the tissues must somehow respond unevenly, in accordance with the shape of the image projected, which is skewed by the lens correction.

I, too, would love to see the detailed biological explanation discovered. But I don’t think we’ll get it until after we get the full myopia one.

At least we have the optical explanation, though. Half the battle.

Speaking of eggs…

The sclera or the choroid…which came first? :wink:


Not sure how this should work with minus cylinder. The spherical equivalent would increase the overcorrection, and so by this theory, the optometrists who use minus notation would reduce cylinder, not increase it. This is not what we observe though.

Also, if accommodation is allowed, a little over-minus will not reduce image quality. So even with plus notation, this problem wouldn’t happen in most cases. The opto would need to use eye drops to disable accommodation, plus cylinder notation, and be bafflingly incompetent. While all three of these happen, they shouldn’t happen together all that often.

My astigmatism corresponds to a different theory: the average cylinder error due to lens distortions outside glasses’ optical center. Relative to the optical center, my glasses are larger on the lower-outer areas. The additional lens error from those areas would induce cylinder error with axes pointing diagonally from down-in to up-out, assuming that I use my eyes roughly evenly within my glasses’ field of view (which I believe I do).

These “happen” to be just the astigmatism axes I have. It looks like my eyes simply averaged the error of my glasses and adjusted cylinder to match this. Depending on head posture and eye habits, the eyes of different people may see different average errors from glasses’ lenses, and thus lens-induce different astigmatism, especially if they don’t use their eyes symmetrically/isotropically.

If this theory holds, it might explain a lot of astigmatism that appears after wearing strong minus spherical glasses. If a spherical lens can lens-induce some cylinder, and the opto madmen then add extra cylinder to “compensate” – we get the good old runaway feedback loop again.

And there’s probably Forrest’s stuff on top of all this, which complicates things because its effect could push astigmatism in the opposite direction.


I got hung up on this idea initially, too, and I had to really think through it before I posted my initial post above.

Here was my thought process:

-That is indeed not what happens when minus cylinder is prescribed, overall. However, after seeing data of a lot of people, here is what I found. Usually when minus cylinder is prescribed for the first time, the minus sphere either stays the same or increases slightly. That means the initial minus cylinder is added under the catch-all category of “myopia”.

-Doctors working in plus cylinder notation seem to give cyl more readily for whatever reason (I have my own ideas as to why, but the quote above from the manual is pertinent in this regard, too).

-In many people, the cyl and sph actually stay about the same over time (“stable vision”). They may alternate back and forth a bit over time.

-When data from our more aggressive (overcorrecting) optometrist friends was looked at, you actually see the pattern I outlined above, in which the cylinder and sphere both increase over time. Often, in real life, the cyl does not increase quite to the magnitude as predicted by the spherical equivalent, but it still increases. This means more of the new error is being converted to “spherical myopia over time”, but the cyl is still keeping pace with or partially keeping pace with the overall magnitude change.

-When plus cyl is converted to minus cyl notation in these extreme overcorrection cases, you might indeed see a decrease, albeit a slight one, in the magnitude of the minus sphere when more cyl is added. The thing to note is that it usually happens in very small increments, there is a back-and-forth over time (stealing from Peter to give to Paul type of deal), and the big one is that most people don’t end up with three or four diopters of cyl over time. But for the the minority that do, and who started out with none, the general patterns of how it increases in accordance with the mathematics posted above (at the maximum, sometimes less) do seem to hold.

-People complain about headaches and warped vision in these cases. Switching doctors (even in the same practice) because they’re fed up often results in a decrease in both sph and cyl.

-Finally, remember the way the optometrist/technician and patient interact during autorefraction, subjective refraction, retinoscopy, etc. all vary by individual and circumstance.

This post was just intended to be one idea, illustrating how the math works in theory. Nothing about blur signals, biological adaptations, etc. I’m sure all the pieces fit together and interact to produce the final result.


Absolutely. This is probably a great potential explanation of how the initial, small cylinder error comes about…that bit that doesn’t need correction, but which nearly everyone has at some point in life.

As for your idea, I definitely notice off-axis errors from glasses, and I prefer smaller frames. It definitely could be a potential contributor; I agree. Sometimes I get so fed up with my glasses and can’t get clear vision anymore without taking them off and resetting for a few minutes.

Re: Forrest. I agree reading tires the focusing in one axis more than the other. Sometimes it seems to “get stuck”. Directional blur results.


Another thing to consider is, perhaps after a while, the error gets “baked into” the brain and/or eyes, meaning it’s harder to get rid of…just like spherical myopia. Minus cylinder is just another vehicle that increases myopia over time…

Imagine a case where the minus cylinder fluctuates up and down over the years between -0.50 and -1.00. But when it fluctuates down, 0.25DS (or more) is added to the overall correction. This happens, too.


If we consider egg shaped eyeball logic its pretty simple (guesswork of course…)
My CYL / SPH ratio is 3/5 = 0.6
So if we assume that the ratio stays the same, we can calculate how much CYL i would have needed when my refractive error was equivalent to 0.25D before i needed glasses:
0.6 = cyl / 0.25 => cyl = 0.6×0.25 = 0.15D which would be negligible.

Dont know if it correlates with any real world data


@itamar, I will say that Jake promotes working the cyl a bit as the sphere decreases over time, to prevent that cyl:sph ratio from getting even higher. I totally agree. I also think this way of reducing lines up better with how we get into higher astigmatism gradually/naturally over time.

Example: If you have -5.00 D of sph and -1.75D of cyl on top of it, if you do nothing about the cyl, but manage to reduce the sph over time to -2.00, but the cyl stays at -1.75, you have a lot worse astigmatism by that time…right?

I think that’s part of what you were bringing to our attention.


So, then all you need is a “madman” opto (thanks, Varakari) or a patient who is unsure “which is better, 1 or 2”, and you’ve increased.

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One word: Autorefractors


Hmm, not sure about this. If you indeed improve by axial shortening, reducing the vitreous chamber by 1 mm or so does fairly little to the cornea, the main cause of astigmatism.

It seems plausible to me that the interaction between myopia and astigmatism could happen through lenses, and so reducing astigmatism becomes more feasible when myopia has already improved, because the lenses worn are less crazy. But this is mostly a guess; I don’t have enough data on cylinder improvement, even just anecdotal, to push ideas with any confidence.

I don’t understand what you mean. An autorefractor might over-minus, but that should be noticed quite quickly. If it has a keratometer, that shouldn’t create any bias to adding extra cylinder.

Except for the general, dangerous mainstream optometry idea to always correct for everything. Given any cause of additionally induced refractive error, adding more correction without identifying the cause will probably just induce more and more of the error. It’s shocking that optometry can ignore this obvious problem without batting an eye.

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Exactly. They read the autorefraction (which often has too high a cyl indicated in addition to sphere), throw that into the phoropter, and use it as their first lens the person sees during the subjective. Every single time I go for an exam, for example, a cyl is put in front of me, and every time the view with it looks worse, so I tell them. The one time I said “no difference”, I ended up with a cyl rx (this was back in high school) and I told the doctor I’d like to go without it. He ended up doing spherical equivalent instead of cyl.

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It hurts to only read this!
I have a friend, low myope, wears glasses for driving (0.75 R&L). Lost glasses. Went to opto (that b$&)&d didn’t listen to me, told him to talk to me before he goes).
Came back with new glasses: r 0.75, l 0.5aph & 0.5cyl…
He presented his glasses to me and it just took me a look till he handed them back to me, letting me bring them back to opto and request 0.75 r/l again :rofl:
Sigh, those low myopes who don’t really care… maybe he’ll get a pair of 0.5s as a birthday present, but not sure if it’s worth it :rofl:


Did just that for my previous girlfriend, who wore -1.25 sph OD and OS. A nice optometrist (who she almost ran off with) prescribed her -0.75 after losing her glasses for the millionth time (and therefore driving with sunglasses at night), but she thought 0.75 was too weak. I asked her if she wanted -1 or -0.75. Said -1. I got her a pair of -1s as a gift. Would not be surprised if she is still using those to this day.

Have a good friend who wears -0.50 to -0.75 with a bit of cyl. Have not seen him in a while, but he had that beat up pair like 15 years. Missing one nose pad and everything! I was like, you know you can get new pads. And btw here’s how to keep your vision from getting worse…