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I encountered this question and has put me into thinking that, "why does not our eye lens have any aberration?". I seem to be confused by optics. I also found that 'since lenses are made up of prisms, thick lenses have a greater angle of the prism than that of thin lenses'. Is that the answer?

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  • $\begingroup$ You're on the right track. Think about Snell's law. $\endgroup$
    – PM 2Ring
    Commented Nov 8, 2020 at 11:14
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    $\begingroup$ The eye does have a lot of aberration. The retina and the rest of our nervous system deals with the problem. $\endgroup$
    – user137289
    Commented Nov 8, 2020 at 23:17
  • $\begingroup$ @Pieter That's true, but the OP is specifically asking about chromatic aberration. I guess the implied question is why don't we normally see coloured fringes on everything. $\endgroup$
    – PM 2Ring
    Commented Nov 9, 2020 at 15:25
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    $\begingroup$ @PM2Ring It is also true for chromatic aberration. Processing by neurons takes care of that. $\endgroup$
    – user137289
    Commented Nov 9, 2020 at 17:04

2 Answers 2

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  1. The lens of our eye has aberrations, a lot of them, chromatic included. If we would take a photo of the image on our retina, it will look pretty horrible. We do not "see" those aberrations because the retina and the brain process the image so to hide the aberrations and present only a clean image to our subsequent processing and consciousness. We are interested in looking at objects, not at aberrations, so they are corrected/compensated by image processing. We do see chromatic aberration from a telescope, a photo or underwater, because this aberration is much greater than what our retina is used to deal with.

  2. By "thin" lens, we mean a lens with thickness much smaller than the diameter; not with small absolute thickness. For example, a lens of 1000mm diameter, 10mm thick, can say that is thin. On the other side, a lens 10mm diameter, 10mm thick, is not thin.
    The eye lenses are not "thin": the cornea is pretty curved; and the crystalline inside is more or less a ball.

  3. Chromatic aberration depends only on material properties (and aperture). A super-thin lens will have chromatic aberrations too, same as a thick lens. Your reasoning of "thick prism" makes sense, but, when you make the prisms thinner, also the power of the lens decrease. Given a fixed material (a glass, or our cornea) the chromatic aberration is proportional to how much the light gets bent. So, a thick and a thin lenses have different chromatic aberration, but also different power. But the ratio Chromatic/Power is the same.

  4. Chromatic aberration of the eye is quite small, specially if compared to other aberrations. This is because the eye has small size, fairly large aperture, large angle of view, and actually it is not very sharp. Chromatic aberration get somewhat lost in the blurriness of other aberrations. Chromatic aberration become important for long lenses with small aperture, like telescopes, because the other aberrations are small, and the pixels are small compared to the telescope focal length, so the tiny chromatic aberration becomes evident. A telescope lens is much more "thin" than the eye, but the chromatic aberration is more significant.

  5. When the lens/prism gets really thick, and light is bent a lot, yes, the chromatic aberration gets proportionally larger, due to the fact that refraction is non-linear. This is evident near the "total internal reflection" angle, where the chromatic aberration becomes enormous. This happens in the eye at the borders of the field (far away from the center), but we have so poor resolution there that again we won't notice.

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You are thinking in the right way!

A lens will not focus on different colors in exactly the same place because the focal length depends on refraction and the index of refraction for blue light (short wavelengths) is larger than that of red light (long wavelengths).

If the lens is thin, the effect will be small because the time is spent on the lens medium is small so there is not much difference in the focus point. But if the lens is thick, the difference becomes clearer. Like in the case of the prism, where one can see How the different color focuses at different points.

As for the Human eye, Its lens is about $4$ mm to $5$ mm (which is considered to be thin). More on this here. There other functions in the Human eye which help it to focus light at the same point.

The lens is a transparent biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. By changing shape, it functions to change the focal length of the eye so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the retina. This adjustment of the lens is known as accommodation. Accommodation is similar to the focusing of a photographic camera via movement of its lenses. More on this here.

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  • $\begingroup$ You should also mention the benefits of the relatively small aperture of the iris, and the small angle approximation, $\sin\theta\approx\theta$. $\endgroup$
    – PM 2Ring
    Commented Nov 8, 2020 at 19:55
  • $\begingroup$ OTOH, it is possible to get significant chromatic aberration when the cornea is not functioning correctly. Eg, if you wear contact lenses (that aren't the modern oxygen-permeable kind) for too long, the cornea gets water-logged, which changes its refractive index, and its dispersion. In that situation, the eye's lens has to work harder, but there's not much it can do about the dispersion. $\endgroup$
    – PM 2Ring
    Commented Nov 8, 2020 at 20:01
  • $\begingroup$ A thin lens has chromatic aberration too. $\endgroup$
    – patta
    Commented Mar 27, 2021 at 13:31

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