What is astigmatism, and why is it a concern? I understand it has to do with how eccentric (variance from a circle) the eye is? Does it distort images, if severe enough?
I don’t rely upon Wiki, and the Mirriam Webster definition did nothing for me. That’s why I rely on the SDope! 
Astigmatism is essentially having different focal lengths along different angular directions (You might have one focal distance for the vertical direction, for instance, and another for the horizontal direction – by the way, the two directions are always perpendicular to each other). Naturally, this causes problems, since it means you can’t focus along both directioon simultaneously. For on-axis astigmatism this is usually caused by some imperfection in the optical system. Even a perfect lens will show astigmatism if used for an off-axis object, though.
They test for astigmatiosm in the eyes by using charts that have objects at various angles, and asking you if any directions are out of focus. Correction of astigmatic vision is done by using glasses that have a “toric” component, so that the glasses themselves have different powers along the different directions. This is part of the yeglass prescription you get from youir eye doctor. Contact lenses can be made to correct astigmatism – the lens is slightly thicker at the bottom. The motion of your eyelid, opening and closing, is what orients them (not gravity). The construction of such astigmatic lenses is fascinating – there are several different methods that have been used.
By the way – you don’t have to rely upon wikipedia – there are other on-line resources.
Sometimes astigmatism is actualy an advantage, and several optical systems have exploited its capabilities. The astigmatically-compensated laser cavity, used for dye lasers and color center lasers is one example.
Now that you’ve been given factual answers, here’s my anecdotal one: yes astigmatism distorts vision. When I don’t have correction, and look at a line of text, i can only see the vertical lines. The horizontal ones blur out of visibility.
Another anecdote: when I first got glasses as a child, I was told that I was nearsighted, and that my right eye was significantly worse than my left. I hadn’t realized before then that I didn’t use my right eye (and suddenly going from 2D to 3D was hella weird).
My prescriptions kept going up year after year (and the right eye’s was always much stronger). Eventually I moved to contacts. I saw several different doctors during this timeframe.
After college, I moved to New Jersey and had to find a new optometrist, and went with the one attached to the local Pearl Vision. At my first appointment, she asked what kind of astigmatism contact lense I used.
“What are you talking about? I don’t have an astigmatism.”
“Yes you do, in both eyes.”
It turns out that my right eye actually has much better vision than my left, but it also has the worse astigmatism. All of the previous doctors had been vastly overcorrecting that eye instead of just giving me the right kind of lenses. The new doctor started stepping my prescription down on that side, and in the process won me as a patient for as long as we both live in the area.
Think of it as looking into a fun house mirror.
For that type of question, wikipedia is way better than this MB.
You may notice that your eyeglass prescription has three numbers: “Sphere”, “Cylinder” and “Axis”. A lens approximating to a section from a sphere corrects short or long sight acccording to whether it’s concave or convex respectively. A lens approximating to a section from a cylinder corrects astigmatism, when rotated to the correct axis. And an actual lens is the mathematical sum of these three magic numbers.
I have to chime in, because it always bugs me to see the “Hey, you could have just looked it up yourself” kind of response.
There is nothing wrong with asking questions here that can easily be looked up. In the first place, you never know what kinds of other answers you might get here; discussions around here often wander into useful or interesting tangents.
In the second place, by asking an easily looked up question here, you might bring it to someone else’s attention who might otherwise not have even thought to go look it up themselves. This question is a perfect example. I was looking over the GQ threads, saw this one and said to myself, “Hey, I’ve always wondered that too”.
So now, thanks to Jinx asking, and CalMeacham answering, and also to you, brewha, for providing the wiki link, my level of ignorance has been just a bit lessened. But if Jinx hadn’t posted, I probably never would have thought to go look up this particular subject.
Just to add a bit to the excellent answers given by CalMeacham and Malacandra – astigmatism can be due to the shape of the cornea (i.e., corneal astigmatism), and non-corneal factors including the shape of the crystalline lens. If most or all astigmatism is corneal, in addition to the toric lenses CalMeacham mentioned, you might be prescribed a spherical rigid gas permeable (RGP lens.) There is a trade-off depending on the type of lens you go with – soft torics are more comfortable than RGPs, at least according to most patients, but can shift or turn on the eye and lead to blurry vision, at least temporarily, and the extent of this blur depends on the magnitude of astigmatism.
I have never really understood astigmatism (until this thread), but I know that I have a butt-load of it, on top of nearsightedness. My eyes are basically useless beyond twelve feet or so.
I doubt that I would have contributed much to human evolution up to a hundred years or so ago.
In optics, astigmatism refers to two different things. The first is what’s been discussed here - the problem of an optical system not having rotational symmetry around its optical axis, for example one of the lenses having a surface shaped like a segment from the side of a football.
The second thing astigmatism refers to describes, even for a hypothetical lens that has perfect rotational symmetry, a rotationally nonsymmetric behavior for a bundle of rays entering alongside the optical axis rather than on it. So, for instance, if you had a nice camera and were taking a picture of a distant building, but held a card with a hole in it in front of the lens, and you put the hole near the edge of the lens rather than on the centerline - why, then, the quality of your picture may be degraded by this kind of astigmatism.
I’ve never heard this second kind discussed in regard to the eye.
>acccording to whether it’s concave or convex respectively
Minor nitpick, but most eyeglasses, and many lens elements in more complicated things like camera lenses, have concave and convex surfaces both in the same lens. They’d both be curving in the same direction, so the lens would be shaped as if it were cut from a thick-walled spherical flask or bubble. Whether the lens has a net positive or negative effect depends on which of the two curves is steeper (has a smaller radius).
There’s a neat and simple principle that often dictates using concave-concave lenses like this. Suppose your lens is tying together object and image such that one of them is much further away than the other (typically in a camera the object is further away, in a microscope objective the image is further away). Draw rays radiating out of the object, and rays radiating out of the image. You can imagine the object rays and the image rays intersecting where the lens is to be. To minimize spherical abberation, which is often the most important one, you want your lens curved, so the rays from the more distant thing strike its convex surface at the same angle that the rays from the closer thing strike its concave surface. The lens wraps, you see, trying to follow the angles of both sets of rays. So a good starting point in simple optical design is a single element whose center imaginary surface is equally steep from the point of view of both things, and whose difference in radii for the two surfaces is sufficient to create the focal length you want.
Astigmatism means that you have an irregular-shaped eye, rather than just too long or too short to focus clearly.
It causes the eye to try and focus in two places at once, which is of course impossible.
If severe, as mine is, it causes hideous blur at ALL distances, and creates massive eye strain and headaches when not corrected.
Of course in middle age, as your close vision fades you then need progressive lenses to correct near, middle AND far distance, which in my case is apparently a very complex prescription to get right.
Contacts I eventually lost patience with, and LASIK surgery wouldn’t work in my case either.
I don’t mind wearing glasses at all, but not wearing them?
Completely out of the question nowadays.
I have some astigmatism, worse in the left eye than the right. Had RK surgery 20 years ago to correct severe nearsightedness (I was legally blind without glasses)
I could have had the astigmatism corrected too at the time in a second operation. Decided not to for two reasons… One, I was tired of having my eyes poked so much… it did hurt badly for a week
But the main reason was that even though my vision was corrected to almost 20 / 20, I could now read without reading glasses, which I never could do wearing regular glasses without bifocals.
Asked the doctor why this was happening and he said the slight uncorrected astigmatism was probably causing that.
Astigmatism can have weird effects.
Speaking of weird things about astigmatism, it’s common among zombies.