Having trouble figuring out my astigmatism

Well, I get that both measurements are converted into diopters of SPH.

I also understand how to calculate CYL with both values.

From my understanding of this statement, the second measurement once converted into diopters of SPH is the SPH value in our “prescription.” If this is the case, it doesn’t really make sense to me. Should not the second measurement always be more cm than the first?

Here’s my example:
1st cm measurement (when few lines blur): 33cm ~ -3dSPH
2nd cm measurement (when all lines blur): 60cm ~ -1.75dSPH

My sph value for OS at that point was -3.5, and the 1st measurement was closer to that.
Basically I was asking if the second measurement shows the SPH value of our “prescription,” and if it did: why?
If you need any clarifications please let me know.

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I actually never took vertex distance into consideration. I’ll definitely look in on it more.

do you currently have a prescribed cyl, and is the value a positive or negative one?

There are two ways optometrists can prescribe:
>> Negative sphere with negative cyl added on top of it (I believe this may be the most usual one, and it matches the example I gave earlier.
>> Negative Sphere with positive cyl, where the negative sphere value matches the weakest axis. In this case, they use a positive cyl value to account for the strongest axis (least prescription needed).

If you fit the second category, I would recommend searching a bit more on the forum. I know this has been discussed, but I cannot think at this moment where that may be.

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My most recent prescribed CYL is
OD: -3.25
OS: -3.50

To clear things up a bit:
Measuring CYL with this method is not 100% accurate for me–actually, most DIY tools aren’t accurate. My measurements are usually off by 1dCYL possibly because my astigmatism value is so high.

Then again, I might be measuring wrong and that’s why I’m getting confused. I remember there was another post explaining much more in depth, how to measure with this tool/what the measurements mean but I can’t find it.

I am thinking at this point there may be a function of both you possibly being confused, and me not having a full picture. We are not supposed to give specific prescription advice, and for many valid reasons.

If still confused and not finding your way, and if this is still an option, maybe see an optometrist for a most current measurement of each eye with each of the shp and cyl values, and you can work your way backward from there. You may still find discrepancies, but may give you a better grip on the numbers.

Another option that comes to mind is the use of a test lens kit. If one is available, do your own measurements, calculate sph and cyl based on that (account for the vertex - maybe 10-13mm of it), put the lens in, and see what happens and how you feel about the result.

I am sorry I was unable to help a bit more.

That’s fine. In no way was I trying to ask for diopter specific advice; I guess you couldn’t understand what I was asking but that could be my fault because I have a hard time properly articulating my questions. I’ll do some more research on the forum and if I find an answer to my question I’ll come back and share.

Are you comparing your results with this method to what your optometrist “prescribed”?

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I think your case is complex enough to look at the full prescription from the opto, not only the cyl values.
This doesn’t mean giving diopter specific advice. Just analysing data and expressing opinion on the full case and then you can make your own decision. :wink:

So I copied your last opto prescription over from your introduction and also what you measured with the astigmatism wheel. And I’d like to note a few things here:

  1. The EM assumption is that the full prescription is typically an overcorrection - both on SPH and on CYL. The unnecessary correction added by optos for eagle eye correction can be around SPH -0.5D and CYL -1.0D.
  2. CYL can be converted to SPH. 1D of CYL converts to 0.5D SPH. This is a conversion formula that optos use, not an EM one. (My opto always does it to avoid the complexity of glasses). Obviously your brain needs to adjust. But if your SPH is -1.00D and your CYL is -1.00D, too, you could wear -1.50D SPH without cyl. This would mean overcorrection on the most part of your eyes and undercorrection on the axis that is different from the rest, but the overall vision could compensate for this over-under part.
  3. Your last opto measurement is from 2019 which - depending on your habits in 2020 and 2021 - may or may not give you full 20/20 or 20/15 correction now at distance.
  4. You’ve had strong CYL added to your glasses for a longer period so it can be assumed that your brain is now used to receiving images where the image of the individual eyes are corrected differently within each eye.
  5. CYL is only added at a certain angle, some people have more than one axis blurry before the full wheel goes blurry. (E.g. I typically have 2 of the lines next to each other going blurry first and a little further away one more goes greyish at a different angle but not in line with the first one, and then all of the lines go grey. With fictitious examples: I may see grey first at 10 & 11 o’clock and the logical “in line” blur would be at 4 & 5 o’clock next, but instead I have the other grey part at 6 o’clock - but then, I have cyl but it never got corrected separately thanks to my opto.)

This is how CYL is added to the overall correction (from Zenni’s page)

Your last opto correction:

And this is what you measure:

Using the 2019 opto measurement of OS(L):
You have -3.50D correction for the left eye in general but at 175 axis you have an additional -3.50D CYL which means you are corrected to -3.50 + (-3.50/2) = -3.50 + -1.75 = - 5.25D at 175 only.
So your opto 2019 is: SPH: -3.50 | CYL: -3.50
and yours for the same eye is: SPH: -1.75D and cyl (2x -1.75DSPH) -2.50D CYL
which gives you a total correction of -3.50D on one or more particular angles (depending on which lines fade away first)

Having read these my main questions are:
a) how well you can see with your existing last opto prescription? And as you have high astigmatism correction, how does it change when tilting your head up-down, left-right?
b) the cyl means at a certain angle of the eye. When optos measure your eyes the text to read is right in front of you or maybe a bit higher up. When measuring at home, you may look a bit downwards to the screen of a laptop?
c) which lines fade into grey first when you measure on the wheel? Optos have a lot of difficulty in deciding how to correct astigmatism, so is your own measurement anywhere near the 165 or 175?
(Since you wore the glasses long, they should be - or the opto was totally off!)
d) do you have a test lens kit to test what you could see with your own measurement? or if not are you willing to order a pair of cheap glasses and see just for the learning?
e) have you considered getting your eyes measured fresh to see what an opto (over)correction would be now? Preferably with an opto who doesn’t have your previous record, saying you lost your glasses need new ones, don’t know the previous prescription, etc?


Yes. From what I understood: if I were to take the second (less strong/more cm) measurement and make that my SPH and at the same time use both measurements to calculate my CYL, that prescription would be really under corrected for both norms and diffs.

Actually, I know you read my intro but I wanted to restate this. I stopped wearing my glasses for four months so I was really blur adapted. The only glasses I had were my 2018 and 2019 prescribed glasses. After being blur adapted for four months, both pairs felt super strong so I went with the 2018 one. At that point, I could see 20/20–probably more—with both. As of now I’m wearing my norms with reduced CYL and it feels the same.

I’ve also tried tilting my head up and down during AF which usually clears up some text. I actually haven’t tried tilting left to right though.

Well I don’t measure with my laptop screen. I printed the wheel out and taped it on a wall at my eye level. Though I try to measure by looking at my eye level or above, I have noticed sometimes I do look downward.

For both of my eyes, the very first line to fade is the horizontal. I’m pretty sure those angles are correct because when measuring with stenopaeic slit, those angles gave me the most clarity.

I’ve considered a test lens kit but I’ve been more open to experimenting with cheap glasses.

I was expecting an opto appointment in 2020 but the pandemic happened (if it didn’t I probably would not have found EM). Since I recently started EM I was thinking of waiting, maybe until a diopter of improvement in SPH, before going to a new opto.

I’d say you didn’t get blur adapted simply, but you actually dropped your overcorrection plus also unknowingly worked with normalised activating AF.
So you had better than 20/20 with this OS[L] | -3.50 | -3.50 which in theory is equivalent to -5.25D without cyl
which suggests that a correction adding up to -4.75D would have still given you a vision between 20/20 and 20/25
and now you measure -1.75SPH -3.50CYL that in theory is equivalent to -3.50SPH without cyl
In theory because with high myopia typically you cannot simply convert cyl to sph despite the maths being correct.

While you can try to order something along this measurement, I think it would be beneficial to monitor the measurement for a few weeks and experiment with tilting the head. If you work in front of a monitor, check it also on the monitor looking at it at the usual monitor angle you work with.
If you still get relatively low SPH with high CYL you should consider dropping from CYL with or without trading in some of the CYL to SPH, too.
E.g. if you constantly measure the same -1.75D SPH with the -3.50D CYL you can try something like -1.75D SPH and -3.00 CYL (cautious drop from cyl only) or -1.75D SPH and -2.50 CYL (a significant drop from cyl only) or -2.00D SPH and -2.50 CYL (drop from cyl and adding back a bit of sph) just to mention a few ideas. I wouldn’t drop from SPH though as you have a big enogh gap between SPH and CYL already.
A word of warning: when you are playing around with change in cyl, it is your brain that has to work hardest to accept that the texts and objects in front of you are now corrected differently so they will need to get processed by the brain differently, too.
It would be super exciting to find out that you indeed dropped so much from the correction within such a short time.
Let us know how it goes :blush:

Thanks for the insight Bianca!

There’s actually a lot of possible corrections I could try so now I’m really debating if I should get a test lens kit. I’ll definitely keep measuring (I want to try different methods at the same time of day to compare results) and see if there are any notable differences. I just hope I won’t make reductions too quickly.

Lets unpack here.

(1) You seem to have the basic understanding of how these measurements work, but are incredulous at the result.

(2) Have you considered the possibility that your opto massively over-corrected you? You wouldn’t be the first here. Optos are usually (mostly) reluctant to reduce and largely (mostly) inclined to give you x-ray vision = ever increasing over-prescription. [There are exceptions, but rare.]

Get good at YOUR measurements so that you are in control of the situation, and not a hostage to ‘official’ prescription(s) from an opto, or any advice from this forum.

A test lens kit in your situation might be a good investment because, your complex prescription (high cyl/sph ratio), and potential sensitivity to cyl changes (50/50 proposition) may present challenges. I was similarly situated (complex) but my astigmatism axes cancelled each other out and I use no cyl, Lucky, but it works – no sensitivity.

Do a forum search on ‘prescription complexity’ and you may find some additional information that suits your situation.

You will have to play with this – astigmatism is a tough one, but you will get there.


I wondered the same things. Thank you for giving my assumptions a backup.

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I thought so at first. Then I made a really drastic reduction for diffs (left me too undercorrected) so I no longer thought I was overcorrected. I basically, thought my “prescription” was 100% accurate. Now, I do think I really am overcorrected but I can’t tell if it’s in SPH, CYL or both.

This is probably the best “advice” I’ve gotten so far, thanks.

I plan to do some measuring for a few weeks with different DIY methods and compare the results–I’ll refrain from comparing results to my “prescription.” I’ll let you guys know what happens.

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When i started EM, I went quickly (2 weeks) from 20/20 to 20/10 or so, which made my old prescription glasses way too strong. When I got my next 20/20 strength glasses I had to drop a full 1 diopter. Going from 20/5ish to 20/20 was a bear. I went in full panic mode for a couple of days, thinking I made a huge mistake (got the glasses from my opto, and they were not cheap by any means). I decided to just give it a few days, and see what happens. In 2 weeks my brain got used to the change, and was already working on improving from t20/20 a bit more.

The point is: a drastic change takes time getting used to, thus all the reasons on why Jake is recommending alternating reducing normalized and differentials.

I cannot speak for the amount of change you are trying to implement, that is purely dependent on your eyes, and specific conditions of your close up work (e.g. preferred distance to your computer). Just keep in mind the thought above for your first pair of differentials.


If you find yourself struggling after a reduction, really struggling with too much blur, getting too tired, never seeing clear, you can always step back. So I’d say measure for a while and then make a first decision and jump!

Hey everyone,

For anyone interested: I just posted an update that explained why I was confused with the instructions of this measuring method and how my measurements have worked out for me in this post. The first three paragraphs basically answered the question I first asked in this thread.

That has been my exact same take on cylinder. I created a spoked wheel in AutoCAD, and it seemed to match pretty close to an old 2011 script I had on file. I just use those same axis numbers and reduce the cylinder power. Nice to see someone else out there that thinks the same way about that pesky astigmatism!


DOH! I spent a minute looking at the image on the left and started worrying why I couldn’t clear the oblique lines and why that axis had gone bad before I realized it’s a “this is what you don’t want to see” example. A rigged test! :slight_smile: