Playing with the astigmatism dial

I was playing with the astigmatism dial (attached) and noticed something.
Without glasses, i cannot get the two lines to be clear simultaneously.
I can clear the line with the arrow up to 16cm and the perpendicular line up to about 23cm.
When i put the dial even closer to my eyes it is easier to clear both lines but not at the same time… when i clear one line, the other lone gets slightly blurry.
Astigmatism sux! :open_mouth:

If anyone can explain what is the mechanism that prevents the eye from clearing both lines at the same time or any other insight, that would be awesome!

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Astigmatism simply means that the eye has uneven power: one meridian (direction of the eye) is more myopic than another.

Even if you are in accommodation range, you can’t focus in a way that befits both refractions. Your ciliary muscle only changes spherical. So if it focuses according to the more myopic meridian, the other one gets hyperopic defocus, since the eye isn’t quite as myopic in this direction. But if you focus on the less myopic meridian, the other direction gets myopic defocus.

So you can’t clear up an image in more than one direction at any distance if you have significant astigmatism. That’s why most people are conservative in cylinder reductions: unlike for spherical, where you can use much weaker glasses for near work, cylinder error can’t really be fixed just by moving closer to your target.

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Fantastic explanation!
Thank you.

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