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Nearsightedness and Farsightedness: Squished Spheres
If you spend a lot of time squinting, it’s likely that you have myopia or hyperopia. Read on to see what’s going on inside nearsighted and farsighted eyes.
As light enters a perfectly round eyeball it passes through a lens called the cornea and gets focused on an area in the back of the eye called the retina, much like a movie projector’s beam hitting a guy standing up in the front row. The retina converts this light into visual information with photosensitive cells called rods and cones—which dispatch the resulting data to the brain, producing a crystal-clear image. But, for a majority of people there’s one hitch: approximately 65% of adults possess misshapen eyeballs that skew the way light hits the retina, resulting in vision problems.
In the case of nearsightedness, or myopia, the eye takes on an oblong shape, causing incoming light rays to meet—and thus focus—at a point just shy of the retina. By the time the rays have actually reached the retina, they’ve begun to diverge again. The farther away the object reflecting light is from the eye, the more pronounced this effect will be, resulting in blurry vision at a distance. In farsighted eyes, it’s just the opposite: the light isn’t focused yet when it reaches the retina (consider a movie projector positioned too close to the screen), although the blurring this produces is less noticeable at greater distances.
These very common conditions have filled medical logs for 2,000 years, before which time everything was bigger so it didn’t matter as much. It wasn’t until the advent of corrective lenses in the 16th century, however, that anyone was able to do anything about it. For nearsighted eyes, convex lenses filter light through a surface that’s thinner at the center than at the edges, giving light rays an extra boost so they can converge on the retina for a clear, clean image. Lenses for the farsighted are thicker at the center, bending the light so that it, too, lands right where it should.