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I have astigmatism. I'm also nearsighted. Without my glasses, I can hold an object (such as reading material) close enough to my left eye to get it in perfect focus, but my right eye doesn't focus perfectly at any distance. The “axis” on my glasses prescription for my right eye is close to 45 degrees, and according to my eye doctor, this is the worst possibility. But I'm happy to know this is just a refractive error that can be corrected with glasses or laser surgery.
Eyeglass prescriptions have numbers for Sphere, Cylinder, and Axis. The Sphere correction is for nearsightedness or farsightedness. It refers to how much the glasses lenses should be convex or concave. The Cylinder and Axis numbers are for astigmatism. Perfect eyes have round corneas; an eye with astigmatism has a cornea shaped like a spoon or the side of a football. The eyeglass lens can correct for this refractive error if it also has a shape more complicated than just convex or concave. Cylinder is a mathematical approximation for the astigmatism correction.
When the Axis is zero or 90 degrees, the cornea is shaped like a spoon held either parallel or perpendicular to the line connecting the two eyes. These are the easiest to correct with glasses. My right cornea is shaped like a spoon held in a diagonal position. Glasses work well enough to get my right eye to 20/20, but still, everything is clearer when I can see it with my left eye.
Adjusting to new glasses is a challenge for most of us with astigmatism. Sometimes straight lines look curved for the first day or so. After I got one pair of new glasses with a stronger astigmatism correction, I thought my driveway looked like it had huge waves. It's an unpleasant sensation, but it goes away as the brain adjusts to the new visual inputs.
Contact lenses can also correct for astigmatism, but I have not tried this myself. Reference books report that hard contact lenses provide the best correction for astigmatism. Soft torric lenses are also used, but are more expensive to fit and to replace.
There are other deviations from perfect roundness of the cornea. See your eye doctor for the latest news on lens options.
Reference:
Clyde K. Kitchen, M.D., “Fact and Fiction of Healthy Vision: Eye Care for Adults and Children”, Praeger, 2007.
Higher order refractive error correction:
http://www.izonlens.com/how-wow-works/
Linda Fugate is a scientist and writer in Austin, Texas. She has a Ph.D. in Physics and an M.S. in Macromolecular Science and Engineering. Her background includes academic and industrial research in materials science. She currently writes song lyrics and health articles.
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