You may remember from way back in your high school days that white light is composed of all the colors of light. Shine white light through a prism, you’ve got yourself a rainbow. Shine white light just right through water vapor in the air, you’ve got yourself a rainbow. Separating white light into its components creates a rainbow because each of those colors was there, but we couldn’t see them as anything but white until they separated from each other.
How does chromatic aberration work?
If you look at those rainbows a little closer, you’ll see they all have the same pattern. The color that is getting refracted (changing course as it hits an interface) the most is always violet, and it is right next to its neighbor, blue. The one that gets thrown off course the least, and is refracted very little, is always red. It has to do with the wavelengths of the different colors. Since light is a wave (and also a particle, but that is a much longer article for another day), it has a wavelength measured by the distance from peak to peak of each wave. The same color will always have the same wavelength.
Short wavelength light is easier to change course. It bends more when it is refracted than long wavelength light. If you shine short-wavelength blue light through a lens, it will bend more than if you shine long-wavelength red light through a lens. And that’s the heart of the issue. White light has all of these colors. So when a lens — any lens — is in the pathway of white light, the colors will be slightly broken up so that the short and long wavelengths of light move away from each other a bit. This is chromatic aberration.
How does chromatic aberration happen in our eyes?
In a camera, and in our eye, these broken waves of light lead to chromatic aberration. The light getting focused on a sensor, or our retina, needs to focus into a perfectly sharp image in order to have no blur. But unless you are looking at a member of the Blue Man Group sitting in a blue chair in a blue room, the wavelengths of light coming into your eye won’t all be the same. Specifically, we are looking at white light and objects illuminated with white light all the time. That’s all the wavelengths of light and therefore all the colors.
So what happens when images try to focus on your retina? The blue light will get focused (refracted) more and therefore end up focusing just in front of the retina. The red light will get focused (refracted) less to end up focusing just behind the retina. This is chromatic aberration inside the eye. The difference in having focused blue light and focused red light is a little more than half a diopter. The upshot of that is this: if you have glasses that are perfect, a blue neon sign will still be a little bit blurry because your glasses are about half a diopter too strong for pure blue light.
This is why you will sometimes see people in highly visually demanding sports wearing yellow-tinted glasses. The yellow lenses block the blue light so there is no chromatic aberration. There is no fraction of the image that is adding a slight amount of myopic noise (because that’s what focusing an image in front of the retina is called — myopia). So the next time you’re looking at neon lights of different colors and wondering why some of them are blurry, you can blame chromatic aberration. It makes all of us a touch farsighted with red light and a touch nearsighted with blue light. And if you see a gun-loving fellow talking about target shooting while wearing yellow glasses, don’t poke fun; because he’s just neutralizing chromatic aberration, and also because he has a gun.