How Does Astigmatism Affect Night Vision & Driving?

How Does Astigmatism Impact the Perception of Light?

When light enters the eye, it is supposed to converge to focus at a pinpoint spot on the retina called the fovea. Astigmatism occurs when, instead of one point of focus, there are two separate points of focus in the back of one eye. This can create blur, glare, secondary images, or starbursts around lights.

Sometimes, this precise focus of light on the fovea moves because of optical changes involving the shape or curvature of the cornea, or the lens of the eye, or a combination of both. The greater the separation of the two focal points, the greater the amount of astigmatism and the greater the distortion of vision.

An Analogy for Astigmatism

Imagine that you are sitting in a theater, ready to watch a movie on the big screen. The lights dim, and an experienced projectionist projects a movie perfectly onto the screen. A nice clear image is before you.

However, at the exact same moment, a novice projectionist also projects the exact same movie onto the screen, and aligns it perfectly, but his projection is out of focus.

As you can imagine, the resulting image would be visually bothersome, but still may be recognizable. This gives you a good idea how astigmatism works. Astigmatism is caused when light does not focus at one point, but instead, it is stretched between two points.

How Does Astigmatism Affect Night Vision?

If your eyes are perfect, lights may glow at night, and the glow tends to be fairly crisp and even in perfect conditions. When a person observes lights at night and notices a starburst (stretching of light effect) over the light that is exaggerated more in one direction than another, like a stretched star (a starburst), they might be witnessing the effects of astigmatism within the optics of their eyes.

Night Driving with Astigmatism

Astigmatism induced starbursts around lights are especially noticed at night, and especially around headlights and streetlights. The lights may appear hazy, or to stretch, or even double, making driving and discerning signs and details difficult.

While driving at night, noticing details as soon as possible is crucial. Every moment counts triple given the limits of our normal visual perceptions at night. Since we travel at speeds up to 75 mph at night, we need our eyes to be as perfect as we can get them to help reduce this loss of vision efficiency at night.

Since we all tend to drive at night, the best way to prevent unnecessary accidents is to first make sure that we can all see our best. As individuals in a community, each of us should get our eyes checked and wear our necessary optical correction for nighttime driving to reduce accidents. If you won’t wear your glasses and see your best, what makes you think the next driver is going to wear their best correction? Safe nighttime driving begins with each of us.

DRIVERS TAKE CAUTION: We aren’t aware of the things we can’t see.

Do Starbursts Around Lights Mean Astigmatism?

If you have Astigmatism, there is a stretching effect around the light (a starburst) that is usually greatest in the direction of the most out of focus focal point within the eye. Therefore, by identifying a stretched image in your vision, you may be able to not only identify that you have astigmatism, but you may also be able to identify the orientation of the refractive error.

Interesting point: if your astigmatism is more out of focus in the vertical meridian, you might significantly improve your vision by squinting (since your eyelids close horizontally to block the vertical meridian).

Even though experiencing starbursts may be a commonly understood way to determine if you have astigmatism, be aware that starbursts are actually a poor gauge for specifically identifying astigmatism. Even people with simple myopia (nearsightedness without astigmatism), simple hyperopia (farsightedness without astigmatism), and emmetropia (no correction needed) can experience a starburst effect. For example, atmospheric effects (such as water or ice crystals in the air), windows, and glasses themselves can create similar starburst effects.

Navigating the Night: Addressing Astigmatism Challenges

Since astigmatism is one of the things that can create stretching and blurring of lights, and can complicate your driving at night, it’s a good idea to get your eyes checked to rule out astigmatism as the cause. Even a person who can read 20/20 and thinks that their vision is good may, in reality, have their vision disturbed by a second offset point of focus,  and therefore have astigmatism. Quite often, this complacence with our vision goes unchecked since we only have our own eyes with which to compare.

Look at it this way, astigmatism occurs in about one third of the population. The rate of astigmatism increases with age, from 14.3% under 15 years old, up to 67.2% in those over 65. So, as age goes up, our likelihood of developing astigmatism also climbs. This change may occur at an imperceptible rate too. If you have any impression that your vision might be changing, or you notice stretching of lights, or that squinting helps make things clearer, or extra blur at night, you may want to consider getting your eyes checked. You might have astigmatism.

What to expect at an eye exam →

How Does Astigmatism Get Corrected?

To correct astigmatism, light just needs to be put back onto the fovea (the most important part of our retina) with a pair of glasses or contact lenses that are designed for it. In glasses, the power and orientation of astigmatism is corrected in a very precise manner. The powers are exactly what is needed to move the points back to the fovea, and the orientation of the meridians of focus are locked into place by the frame.

In contact lenses, your doctor must find a contact lens equivalent that is both close to that precise power and remains stable so the meridians remain in position. Notice that with glasses, the lenses are screwed into place, whereas with contact lenses, the lens has to be designed so that, when it is free-floating in the eye, the meridians don’t rotate. Even a well-fitted contact lens designed to correct astigmatism (often called a toric lens) tends not to focus light as perfectly as its correlated glasses prescription, but they can get very close to the same precise optics.