So if you read my post My First Research Term For The Arcanum: Silence. Why? It's Complicated…, you’d know what this is all about. As Robin Griggs Wood’s apprentice in Trey Ratcliff’s Arcanum, part of each week’s challenge is to research a term that relates to creativity. While the first term I researched was silence, this time, I chose "chromatic aberration.” Whether you know what it is or not, you’ve seen it, you’ve experienced it, and it’s annoyed the living crap out of you, so here’s a basic breakdown of the bugger and what I’ve found.What Exactly Is Chromatic Aberration?
So this is one that has usually just meant nothing more to me than “those annoying purple or green fringes I get on high contrast edges in some of my pictures, especially when the sun is involved.” I knew it had something to do with the lens, something to do with the light angle, and something to do with, well, sciency stuff, so figured this was a perfect opportunity and reason to find out once and for all what chromatic aberration is and how and why it’s caused (and if there’s anything I can do to avoid it).
The science: Wikipedia puts it like this: “Chromatic aberration (CA, also called achromatism, chromatic distortion, and spherochromatism) is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have different refractive indices for different wavelengths of light (the dispersion of the lens). The refractive index decreases with increasing wavelength."
Well, duh! I could have told you that (yeah right…).
My next destination was a link that leads to a Georgia State University Physics page that explained how blue light wavelengths are shorter than red light wavelengths, and since focal length depends on a phenomena called refraction, which is “the bending of a wave when it enters a medium where its speed is different,” the index for refraction blue and red lights are different.
So now that the smart people terms are out in the open, I needed it explained in layman’s terms, so I did a bit more digging and I found this at PhotographyLife.com: "Chromatic Aberration, also known as 'color fringing' or 'purple fringing,' is a common optical problem that occurs when a lens is either unable to bring all wavelengths of color to the same focal plane, and/or when wavelengths of color are focused at different positions in the focal plane. Chromatic aberration is caused by lens dispersion, with different colors of light travelling at different speeds while passing through a lens. As a result, the image can look blurred or noticeable colored edges (red, green, blue, yellow, purple, magenta) can appear around objects, especially in high-contrast situations.
A perfect lens would focus all wavelengths into a single focal point, where the best focus with the “circle of least confusion” is located.”
Aha. And that actually helps it become a bit clearer and more approachable.
So what it sounds like is that it happens when all of the colors of the spectrum can’t make it to their intended finish line at the time the shutter is pressed. Whether it be actual physical limitations, or limitations based in physics and the ways wavelengths move and work, the stars are just unable to align due to factors including exposure settings, positioning of the lens, and how much glass the waves have to penetrate in order to reach the sensor.
Essentially, because of the way that light works and how each color has it’s own distinctive wavelength, in order for them to be accurately represented, they need to be in the same place at the same time, and when you’re taking a photo and each of these wavelengths of color has to travel through different, it makes it very difficult for them to reach the same point at exactly the same time. So some of them are left on the fringes. They ‘re not usually very apparent, but when you’re dealing with high-contrast edges, it’s easier to see them because of those hard edges. Almost the same way it’s easier to see the immediate difference between black and white versus a gradient that moves from black to white gradually. Imagine an image that was half black and half white, if you put a gray line separating them, it’d be extremely apparent on that hard edge. Now, imagine an image that was a gradual gradient from black to white, and you put a grey line somewhere in between, it’d be a bit harder to pick it out because it’s lost in a gradient of grey - there’s a lot of grey area in between. That’s essentially what’s happening. Some of the wavelengths are getting caught in between the point of origin and the destination - your sensor, like on the black/white card.
Now that I have a clearer idea of what’s happening, I was curious if there was a way to avoid it, and apparently there isn’t. At the moment, science and physics trumps the human mind’s capacity to solve this one. That same PhotographyLife.com link explains that "This kind of...chromatic aberration is present even on high-end, expensive lenses like the Nikon 35mm f/1.4G.”
Luckily for us, we do have a go around, one I’ve been using for quite some time, and one that apparently I’ll continue to have to use, at least for now. Lightroom has a chromatic aberration correction tool in the ‘Lens Correction’ tab on the ‘Develop’ pane. You click it, and then there are sliders for green and purple that you can slide to determine the color range of chromatic aberration to remove, and, wallah! It’s gone!
In this case at least, software trumps physics…