Using astigmatism to find the edge of blur when measuring centimeters

Hi all,

I am using the Diopter Calculator on Android to measure my centimeters to blur. However, often I find it difficult to decide where the edge of blur is, so my measurements are varying wildly.

The difficulty is that some letters, or parts of letters, may start looking blurry while other parts are still clear. I think this is due to my astigmatism (-0.75 in each eye). For example, when I move the phone away, in the letter O in FOCUS the top and bottom will get blurry before the left and right do; the letter U stays clear longer than the letter C for the same reason.

But perhaps we can put this to use. What if instead of the word FOCUS we draw a large cross of two thin straight lines perpendicular to each other. Ideally, the cross can be rotated such that one line is parallel to the axis of astigmatism, and the other one perpendicular to it. Then, if I move the phone away, at some point one line will become blurry while the other line is still crisp. Looking at two perpendicular lines drawn on a piece of paper, I found it very easy to find this point, because the crispness in one line and the blurriness in the other can directly be compared to each other in the same field of view. If I increase the distance further, both lines become blurry and I know I have gone too far.

The distance at the point where one line is crisp and the other one blurry should then correspond to the spherical equivalent of my full prescription (spherical + cylindrical / 2).

Does this make sense, and has anybody else tried this? I am just wondering if using a cross could provide a more accurate way of finding the edge of blur than looking at a word like FOCUS.

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You might also try using something like this

or

LCD Visual Acuity Chart 18

These images are designed to test astigmatism and should make it easy for you to figure out at what distance it kicks in or asserts itself.

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