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Can someone simply explain astigmatism and its different types?

How are they caused in lenses, and how can they be identified in images?

I'm looking for a simple, understandable explanation. I've read the Wikipedia page multiple times, and I don't understand astigmatism, its causes, or its effects. I have no problem copying and pasting, but no way I'd be able to explain it to a fifth grader without waving my hands and saying "just read the Wikipedia page".

mattdm
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xiota
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  • What exactly is unclear after reading the introduction in the linked article, especially "If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances." Are you looking for a real world image given that there's only a typography based example image in the article? – null Feb 11 '19 at 15:20
  • The sentence I quoted appears to be pretty simple. A fifth grader very likely knows what a cross is. Assuming that the concept of focus is known, the explanation that astigmatism causes different focus on the vertical and horizontal parts appears to be somewhat understandable. The example appears to be taylormade to illustrate the key concept of astigmatism. Hence my comment: how much more simple do you expect an answer to be? – null Feb 12 '19 at 20:28

1 Answers1

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The task of the lens-maker is to fabricate a lens that projects a faithful image. Only one glass lens element will project an image, but that image is flawed. This lens will be fashioned convex – convex. This shape resembles a lentil seed hence the name lens. This is because the job of the lens is to gather light rays and then redirect them so that an image is formed on the flat surface of film or digital image sensor.

Alas, every lens ever made is flawed. The optical world is plagued by seven major lens errors called aberrations. The lens maker strives to eradicate each aberration, but, so far, this has proved to be impossible. The best we can do is to greatly mitigate each one. Technically is will take seven lenses working in consort. Some of these will be glued to partners; some will be air-spaced. Some will be made from dense glass, and others from lightweight glass.

The basic shape of each lens element forms a segment of a sphere. Some of the lenses in the group will be convex (bulge outward) some concave (curved in). Try as we might, the shape (figure) of the polished glass surfaces will have some flaws. A common flaw is an error in the steepness of the curve. When the steepness is different left-to-right as opposed to up-down, a circle will image, not as a perfect circle but slightly squished (oval). This is the classic indication that the aberration called astigmatism is under-corrected.

The up-down vs. left-right figure inconsistency can actually be in any direction like slanted etc. When imaging graph paper (cross lines) horizontal lines vs. vertical lines will image with different degrees of sharpness. The shaper lines display increased contrast thus the upright of a cross might image more distinct than the cross piece.

Alan Marcus
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  • Could astigmatism could be the cause of oval-shaped bokeh where the long-axis of the ovals appear to radiate from the center, as in the sample image in this question? If not, what would your conjecture be? – xiota Feb 12 '19 at 17:20