It is still difficult to measure astigmatism even with a test lens kit

Are there any tips from you to find out the right cyl correction?

I still have doubts about my cyl even though I measured it using a personal test lens kit. sometimes when I add next 0.25D cyl I get a reduced blur but the blur is reduced like a spherical blur. so I am confused how much cyl correction I am currently having.


If you want to see how optometrists are trained to measure CYL (and SPH) using trial lens kits, have a look at:

A key tool they use in this context is the Jackson Cross Cylinder (JCC):


I like the stenopaeic slit method better. It became immediately obvious why I was having trouble with cylinder, my astigmatism wasn’t cylindrical. In the one axis everything was clear, in the perpendicular axis I saw a dozen ghosts, not blurring.

I’m no expert on (regular) astigmatism, but I think it would be expected to manifest as spherical blur when the cylinder correction is wrong. I tried to explain (my understanding of) it on the wiki page

The focus is smeared out along the line of sight, rather than a single point, so there’s myopic directional blur in one axis, and hyperopic directional blur in the orthogonal axis, giving a net circular sort of blur. As cylinder correction is changed towards the optional value, the amount of blur reduces.

It manifests as directional blur only when the object is too distant to keep the “circle of least confusion” (?) on the retina.

There’s scope for an experiment, though it might not be so easy if the stenopaeic slit is part of your trial lens kit… If you orient the slit so that you see a single image, then move it back and forth in the orthogonal direction, you might see the image jump between the location of the ghosts.

This would be the analogue of the pinhole diagnosis mentioned on

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