Astigmatic images / Interval of Sturm
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If I had a +1.50 -0.50 x 180 lens outside in the nice bright sun I would
not get a round image of the sun that I could use to start a fire. Instead,
when I held it one meter from the paper I would have a line image, and
when I held it 2/3 meter from the paper I would get another line image.
What would I have in-between these two distances? I would have a varying
amount of fuzzy image, going from the first clear line, to elliptical in
one direction, through round, to elliptical in the other direction, to
the other clear line.
The position of the lens to the paper that results in a round [but not
focused] image will be the focal length for the spherical equivalent of
the lens. This results in an image with no distortion. How out-of-focus
the image is depends on the astigmatic interval, or how much cylinder the
lens has.
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thanks to 
Copyright Ellen Stoner, 1998