Eye Problems

Why does astigmatism make lights blurry?

I don’t know who the marketing guy is for astigmatism, but he’s amazing. Same for the person behind antioxidants. A masterclass in putting a word into...

blurry city lights
Joel Hunter, MD
Joel Hunter, MD
Refractive Surgeon, Hunter Vision Updated 12/17/20 10:00 AM

I don’t know who the marketing guy is for astigmatism, but he’s amazing. Same for the person behind antioxidants. A masterclass in putting a word into the public consciousness and keeping it there. Astigmatism and antioxidants both share another feature. Despite the “stickiness” of the word in our minds, they remain vague and shadowy, their true nature unburdened by specifics.

But today we’re here to talk about astigmatism. We’ll save antioxidants for another day.

If you’ve got astigmatism (and chances are really high that you do), you may have heard your eye is shaped like a football. I spent a couple decades confused by that description until I became an eye doctor. Maybe you need to like sports more than I do for the analogy to hit home. Then I bet the analogy is “slammin’ long bombs away.” (I really don’t follow sports.) In case you’re like me and the football analogy is unhelpful, here’s another way to look at it. Bonus: it will explain why lights are blurry!

Imagine you’re holding a two foot by two foot square of plexiglass, one hand on each side, right and left. If you hold it gently without squeezing it, the plexiglass will be flat. If you look at something through the plexiglass, it will be clear and undistorted. Now imagine you squeeze in with your hands. You’re curving that plexiglass, creating a hill, between your right and left hand. Top to bottom, it’s still perfectly flat, but side to side, it’s got a curve to it now.

With that squeeze, you’ve given the plexiglass astigmatism. That’s all it is. It’s curved in one direction (in this case horizontal) and flat in the other (vertical). If you look at something through it now, it’s going to be distorted like a funhouse mirror. That distortion will become even more prominent with a point source of light, stretching it into a long weird line with a bright spot in the middle.

When you’ve got astigmatism on your eye, the clear window on the front (the cornea) is like that plexiglass. The cornea is also a really powerful focusing lens. So just the slightest bit of “squeeze” (aka astigmatism) has noticeable distorting/blurring effects. Where can you pick up on that the most? Point sources of light! You’ll find those at every streetlight, car headlight, and LED clock near you.

Some folks — and you may be one of them — have a mild amount of astigmatism. Sometimes that’s true even with glasses or contacts because their prescription isn’t quite perfect. But with mild enough astigmatism, it’s undetectable during the day. We only see it when the lights come out at night.

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