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Smartphone imaging sensors are usually Bayer sensors which have a color filter array on top of each pixel, so each pixel ultimately captures one of red, green or blue colors of light. In that case, how can chromatic aberration take place in images captured with smartphones? I know I'm missing something here but can't figure out what it is.

My understanding of chromatic aberration is that red, green and blue colors of light bend differently owing to having different wavelengths, but what I don't get is how the phenomenon can persist when there is a color filter array on top of each pixel ensuring that light of only one color passes through it.

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  • Can you first say what you mean by smartphones suffering degrees of aberration? Jan 7 at 23:22
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    If the filters only allowed R, G, B colors to get through then other colors such as Y wouldn't get through. Each filter has to allow a band of colors through. ( When the colors are reproduced by printing/displays then all colors can be synthesised from R, G, B. But that is the minimum. Many printers prefer CMYK. Canon imagePROGRAF PRO uses 11 colors of ink to make more realistic colors).
    – QuentinUK
    Jan 9 at 17:18
  • Your understanding of Bayer array filters is fundamentally wrong. 1) They don't allow only one color (or even only one band) through. They have overlapping transmissivity with the other two filters, just like our retinal cones do. 2) They're not actually red, green, and blue. The colors of the Bayer mask are closer to the maximum sensitivity of each type of human retinal come than they are to the RGB colors of our emissive displays. For more, please see: Why are Red, Green, and Blue the primary colors of light?
    – Michael C
    Jan 9 at 17:59
  • See also: Why don't mainstream sensors use CYM filters instead of RGB? (Hint: they actually something more like Yellow-Green-Violet than RGB) and RAW files store 3 colors per pixel, or only one?
    – Michael C
    Jan 9 at 21:19

6 Answers 6

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Because the chromatic aberrations happen in the lens, not the sensor - the picture is already distorted by the time it reaches the sensor, so it doesn't matter if there is a Bayer matrix (or any other kind of matrix. Or no filter at all) in front of the sensor or not.

(As an aside, there is no difference between a smartphone and any other sort of camera here)

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    Re, "bayer matrix...or no filter at all." Indeed. Chromatic aberration is detrimental to the image quality even if you're shooting on black and white film. Of course you won't see the colors of the fringes in a B&W photo, but the fringes still are there, and they soften the focus of the picture. Jan 6 at 16:16
  • If you had a same single wavelength filter in front of the whole sensor matrix, chromatic aberration would disappear.
    – jpa
    Jan 9 at 15:14
  • Note that with silicon sensors, there's some chromatic aberration in the sensor itself. The mean penetration depth is greater in the red than in the blue. Photons coming in at a slant are detected at some displacement from the point where they enter the silicon. Because of this, the effective plane of best focus is deeper in the silicon for red light, and it's not possible to focus as well in the red as in the blue.
    – John Doty
    Jan 9 at 15:19
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First chromatic aberration is "function" of lens, not sensor. And I have no information about lens which do not have such aberration. Moreover because of the size lens in phone cameras have less space to correct it.

Second the image from camera (not RAW) is fake in sense of colours. This is because each pixel is generated by camera/postprocessiong software based on specific formula/algorithm from this and surrounding pixels. The only exception I know is image from Sigma Foveon sensors which actually have 3 sensors with filters per pixel.

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    Well, Foveon's colors are also fake, because spectral sensitivities of the raw color channels are not linear combinations of LMS sensitivity functions.
    – Ruslan
    Jan 6 at 22:10
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Chromatic aberration is a plague suffered by every lens. Opticians can mitigate but not eliminate. The simplest solution is to construct the lens using two lens elements. One is a strong positive lens convex as to its figure. This lens is too strong thus the focal length is too short. This lens is sandwiched together with a weak negative lens with a concave figure. Both exhibit opposite chromatic aberration so they tend to almost cancel the chromatic aberration as they adjust the focal length. Unchecked aberration – red rays have a longer focal length then violet, all other colors come to a focus at some intermediate distance.

Now image size is function of focal length. The further downstream the focus, the larger the projected image. Thus, the red projected image is the largest, violet the smallest, all other colors form an image that is intermediate in sized, between the two extremes.

Again, chromatic aberration can be mitigated but not eliminated. The longer the focal length the greater the disparity of size of the various superimposed colored images.

We see this aberration as a color fringing most visible at the borders of the images of object. All and all, there are 7 major lens aberrations. They can be mitigated but not eliminated. There are two types of chromatic aberrations. One produce superimposed image each with a different magnification (size). The other shifts abnormally the various colored images causing them to fall off target.

Nevertheless, modern lenses can be and usually are well corrected thus residual uncorrected aberrations are reduced to tolerable levels.

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At a basic level, the software that processes the data from the Bayer sensor is trying to reasonably faithfully reproduce the analog image that was projected onto the sensor. This would include chromatic aberration, as the software doesn't really know what you are taking a picture of and what may or may not be wanted image details.

But the software can know what chromatic aberration looks like, and can offer a "Remove Chromatic Aberration" as a checkbox or command (Lightroom does this).

Going further, cellphone cameras already tend to use more automatic post processing and they presumably know the characteristics of the lens being used, so it would not surprise me if most cell phones did this automatically. But I don't know this for a fact.

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Addendum: There is a false assumption "light of only one color passes through it.". The materials sensor color filters are made of attenuate other colors to varying degrees, but they are far from "brick wall" filters that work on a "650nm, all gets through. 650.01nm, nothing gets through" basis. Not only would making anything near that be likely technically impossible within the constraints of a filter that needs to be so thin, light and small as to fit on a sensor pixel, it would likely create very confusing results given the human eye also does not perceive colors that way...

In any case, the answers that point out it is about the lens (as soon as the lens creates chromatic aberration that exceeds the size of a pixel, it will simply be recorded by the next pixel) are of course 100% correct.

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I think a possible source of confusion here is that colour is continuous not discrete - it's convenient to talk about "red light" as though it had a single definition, but in reality, "visible light" can be composed of any combination of the entire spectrum of wavelengths that our eyes are equipped to detect.

So when you say:

... each pixel ultimately captures one of red, green or blue colors of light ...

You are missing that our colour vision works not by picking three points on the spectrum of wavelengths, but by comparing three overlapping sections of it. We can trick the eye by displaying three discrete wavelengths, which match the peaks of these categories; but to capture a scene, we need to record what the eye would see looking at it. So even on a digital camera with an explicit "RGB" filter, there are a wide range of wavelengths being captured.

Similarly, when you say:

... red, green and blue colors of light bend differently ...

You are missing that there are not three choices of "bendiness" that light can choose, but a continuous function where any wavelength you pick has its own distinct focal length. So any scene projected through a lens has a whole array of different wavelengths, all focusing slightly differently.

Put those two things together, and you'll see that the filter in front of a digital camera sensor can't correct for chromatic aberration, because the "red" sub-pixels still need to capture both the "reddish-yellow" light and the "yellowish-red" light, which are focussed slightly differently.

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