# What determines whether chromatic aberration will be purple/green or red/cyan?

In my experience, both with my own photography and looking online, chromatic aberration is usually purple on one end and green on the other. However, I have also seen the same effect with red and cyan, such as in this question.

It occurs to me that these are both sets of complementary colors; indeed, both purple/green and red/cyan are used for anaglyph 3D for that very reason. Does that mean that you could also get a yellow/blue CA? What about the lens determines the colors that are visibly fringed?

When considering longitudinal CA, one must see the range of colors as a linear spectrum, rather than a circular color wheel. Infrared light with its very long wavelengths is on one end of the spectrum, ultraviolet with a very short wavelength is on the other. In between you have the visible spectrum: red, orange, yellow, green, blue, indigo, and violet in that order.

If a lens is optimally focused for the center of the visible spectrum (green), then the colors closest to the extremes will be the most affected by longitudinal CA (red on one end, purple on the other). If the lens is focused more for the blue wavelengths then the red on the opposite end of the spectrum will be most prominent. If the lens is focused more for the yellow wavelengths, then the purple will show up more. If you are seeing a lot of red and purple CA, then you are probably seeing longitudinal CA.

With transverse CA it depends more on which colors are brightest in the scene. If you are noticing a lot of green CA, then you are probably seeing transverse CA.

Yellow and blue are closer to the middle of the visible spectrum on either side of green in the middle. It is not likely you would ever see that combination of any type of CA, and almost certainly unlikely to see longitudinal CA in that color combination.

This shape together with the hardness of the lens causes light rays to be redirected, they are bent inward as they transverse the lens. This action is called refraction. The light rays are caused to trace out a shape that resembles a cone of light. The length of this cone is measured when imaging a far distance object. This distance is labeled the focal length.

Now the distance lens to the apex of this cone is a variable. The cone length is shortest when imaging an object said to be at infinity. The cone length or the back-focus distance is elongated when imagining object that are closer. Thus we must focus our camera by adjusting the distance lens to imagining chip (or film), based on subject distance. We adjust and cause the apex of cone to just kiss the surface of the chip or film.

The length of the cone of image forming rays is additionally complicated by the fact that each color of light is refracted inward at a slightly different angle. The result is, violet comes to a focus closer to the lens and red come to a focus further downstream. Thus each color has a different back focus distance and the magnification realized is a function of this projection distance. Translated, the violet image is smaller than the red image. The size of each of the other images will be different as green, yellow, and orange focus all come to a focus at intermediate distances. This is called variation of focal length more commonly called longitudinal chromatic aberration. What we see is a series of superimposed images. We see this as a rainbow of colors that surrounds objects. Since red forms the larger image, red occupies the outside of the color fringe.

There is a second type of optical color error. This one is called transverse chromatic aberration. This causes the focused image points called circles of confusion to fall at slightly different positions. These we call circles of confusion because they overlap and have indistinct bounders. This aberration causes the circles to appear as varicolor patches. Sometimes the red circles stand because their intensity is higher. Sometimes the violet circles stand out because they have higher intensity.

The lens maker attempts to migrate chromatic aberrations but so far, they have never been eliminated. An exception is a surface mirror lens. These are free of chromatic aberrations as the image forming rays reflect off the silvering thus they do not transverse glass.

• Silver first-surface mirrors are unique as they reflect all visible light equally unlike other metals.
– Stan
Commented May 28, 2016 at 3:00

https://photographylife.com/what-is-chromatic-aberration

I found this article. Apparently the color changes once you past the focal point in the photo. In the foreground it's violet (just for Stan) and green in the background. Hope this helps.

• Colour Nazi: Violet is a spectral hue. It can be found in the electromagnetic spectrum at a specific wavelength. Purple is a hue produced by a pigment that absorbs light selectively. Purple cannot be found in the spectrum.
– Stan
Commented May 28, 2016 at 2:54
• @Stan True only within a specialized language domain. In English, it's not so precise. Merriam-Webster gives violet as "any of a group of colors of reddish-blue hue, low lightness, and medium saturation", while purple is "any of various colors that fall about midway between red and blue in hue" (making violet a specific low-lightness, medium-saturation form of purple). Commented May 28, 2016 at 7:37
• @Mattdm - I'd accept that reference in a language discussion. Here, we are talking about an important elemental difference between stimuli within a specific scientific/technical context. Within that context, purple as a hue can be mixed by combining red and blue pigments whereas violet cannot be mixed by any means. Purple has chromatic impurities. Violet does not. Of course, when discussing mere colours, one can blur (!) the vital distinctions between the two. For the sake of clarity, I am stressing that the two are not just different colours. They are unrelated in any but superficial ways.
– Stan
Commented May 28, 2016 at 18:54
• Violet is shortest wavelength visible to humans, it ranges from 310 thru 450 manometers. Dictionary definitions mainly deal with paints or dye or pigments describing how they are mixed to yield a color that resembles the violet we can see at the short frequency end of the visual spectrum. Commented May 28, 2016 at 19:50