Equalizing astigmatism

I have about 0.75D of astigmatism difference between the left and right eye.

From reading the blog my understanding is:

  1. First work on eliminating astigmatism from Differentials
  2. Jake recommends to have the same L vs R difference in all glasses we use

So i am a bit unsure as to what would be the best approach:

  1. if i eliminate astigmatism in diffs and then i will have a different ratio in my norms which will still have astigmatism difference
  2. convert some cyl to sph and stay with the same difference in diffs and norms?
  3. any other ideas?

I hope this doesn’t fall into diopter specific advice…

IMO, the most important idea is to respect the spherical equivalent as your real myopia diopters.

On average, cylinder diopters count half as much as spherical diopters to determine your myopia. That’s why spherical equivalent is spherical error plus half cylindrical error. So if you cut minus cylinder without changing anything else, the eye has to deal with two changes: it sees more astigmatism, and a bit of extra myopia.

Have a look at how your eyes are doing with your current glasses. Is the eye with more cylinder error ahead? That could be a good time to cut some cylinder. If it’s behind, and you still want to cut cylinder, you may need to add some spherical back, to avoid overburdening this eye with too much error all at once.

Of course, if your glasses are written in plus cylinder, it’s the other way around. So if you cut plus cylinder, you may want to cut some minus sphere as well, to avoid overcorrection.


Thank you for the detailed answer.

My weaker eye, which had -0.75 D cyl more than my dominant eye in the original prescription a year ago, is a little behind.
I can see 1-2 more rows down the snellen with the dominant eye (i am now wearing an equalized prescription)
I think i equalized too quickly, without letting my weaker eye enough time to adjust.
I think what i should do is add 0.25 sph to the weaker eye, and maybe even 0.25D astigmatism so that both eyes will be able to read the same line on the snellen.
(Also, my cm measurements show about 0.14 sph and 0.2 cyl difference on average)

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@itamar, I would advise never adding more cylinder…ever. If you feel you need more minus cylinder, it means you need to work on improving your myopia through your current glasses, or reduce the sphere if it’s already good vision. This will have the effect of making the cylinder a larger proportion of the total power, but without actually increasing it. I hope this made sense.


@FMR That advice is usually good, but I wouldn’t say never to add cylinder. If, for example, someone reduced cylinder way too quickly, it may be smarter to take a step back. Of course, it’s generally smart to be conservative with cylinder while working on spherical as well, but that goes both ways. Wouldn’t want to deal with way too much blur.

From your difference between the eyes, it sounds like a close call. An SE difference between the eyes of 0.24 isn’t so bad for equalizing; I managed to do it with a bigger gap. Have you tried incorporating a little bit of winking or patching, to try help your more myopic eye catch up?

If you’re in the north and like me, winter may have stalled your progress. Personally, I’m going to give it more time and see if my eyes pick up improving again. It took many months until I had notable results in equalizing, and that was in summer with plenty of sunlight.


True. But if it was there to begin with, that isn’t really adding, just replacing what was cut too quickly. I agree with that.

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