What is Chromatic Aberration?

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, as shown below:

Corrected Chromatic Aberration

In reality, the refractive index for each wavelength is different in lenses, which causes two types of Chromatic Aberration – Longitudinal Chromatic Aberration and Lateral Chromatic Aberration.

Longitudinal Chromatic Aberration

Longitudinal Chromatic Aberration, also known as “LoCA” or “bokeh fringing”, occurs when different wavelengths of color do not converge at the same point after passing through a lens, as illustrated below:

Longitudinal Chromatic Aberration

Lenses with Longitudinal Chromatic Aberration problems can show fringing around objects throughout the image, even in the center. Red, Green, Blue or a combination of these colors can appear around objects. Longitudinal Chromatic Aberration can be dramatically reduced by stopping down the lens. Fast aperture prime lenses are typically much more prone to LoCA than slower lenses.

Here is an example of longitudinal chromatic aberration that is visible at different distances:

Focus Accuracy AF Fine Tune 2

Note the green color on the top of the image, transforming to neutral in the middle, then becoming purple on the bottom part of the image that is closer to the camera. This kind of longitudinal chromatic aberration is present even on high-end, expensive lenses like the Nikon 35mm f/1.4G. This type of LoCA / bokeh fringing can be significantly reduced in post-processing. For example, Lightroom 4.1 has the “De-Fringe tool”, which allows one to select an eye dropper under the “Lens Corrections” module and pick a fringe color that needs to be corrected. With such tool, one can either completely eliminate this type of fringing or reduce it significantly.

Here is another example of longitudinal chromatic aberration with green and purple fringing, visible on both sides of the tree trunks and branches:

Uncorrected and Corrected CA

The bottom crop was corrected in Lightroom’s “Lens Corrections” sub-module with a single click. The same can be done in Photoshop, but involves more steps (if not using the Camera RAW tool).

Lateral Chromatic Aberration

Lateral Chromatic Aberration, also known as “transverse chromatic aberration”, occurs when different wavelengths of color coming at an angle focus at different positions along the same focal plane, as illustrated below:

Lateral Chromatic Aberration

Unlike LoCA, Lateral Chromatic Aberration never shows up in the center and is only visible towards the corners of the image in high-contrast areas. Blue and purple fringing is often common on some fisheye, wide-angle and low-quality lenses. Unlike Longitudinal Chromatic Aberration, Lateral Chromatic Aberration cannot be removed by stopping down the lens, but can be removed or reduced in post-processing software.

Here is a corner crop from the Nikon 35mm f/1.8G lens that has a rather severe amount of lateral CA in the corners:

Nikon 35mm f/1.8 @ f/2.8 Corner Comp

Unfortunately, many lenses have both longitudinal and lateral chromatic aberrations present at the same time. The only way to reduce these aberrations, is to stop down the lens (to reduce LoCA) and then fix lateral CA in post-processing software like Lightroom and Photoshop.

While many modern lens manufacturers employ specific techniques to reduce chromatic aberrations using achromatic/apochromatic optical designs and special extra-low dispersion elements, chromatic aberration is still an issue on most prime and zoom lenses that we just have to learn how to get around with. The good news is that many modern DSLRs incorporate special in-camera post-processing techniques to reduce and even eliminate lens chromatic aberrations and plenty of software packages are also capable of dealing with chromatic aberrations.

Comments

  1. 1
    ) Eee
    November 10, 2011 at 9:38 am

    Hi Nasim,
    I have a question regarding lateral chromatic aberration…. whenever I bring an image into Adobe Camera Raw (via Photoshop), I always check for chromatic aberration by magnifying the image to about 200%–as it’s so much easier to spot CA at a higher magnification. On other occasions however, I’ve just converted an image to a tiff using Nikon’s ViewNx, and strangely enough, I rarely find chromatic aberration in these images, if I do find it, it’s usually so minimal it’s really debatable as to whether it needs fixing. So what do you think is going on? Could it be that Nikon’s software is better at recognizing and fixing CA when it’s converting a NEF to a tiff? … Or could it be that Photoshop’s software isn’t able to automatically correct it, OR worse, it’s actually creating chromatic aberration because it doesn’t have access to all of the proprietary info and algorithms used in Nikon’s NEFs? Any thoughts? I wonder about PS causing chromatic aberration, because I sometimes depending on the settings used in Raw–(even just sharpening can make CA worse). Anyway, thanks for all you do, I really enjoy your posts!

    PS… Along those same lines, every once in awhile I’ll have an image that shows some very subtle color banding (rainbow-like) in shadow areas when using Photoshop to covert an image. But yet when I process that same image using Capture Nx, the color banding is not apparent. It makes me wonder with certain “trickier” images, if you are better off using your camera maker’s software to convert the image from Raw.

    • November 13, 2011 at 10:17 pm

      Eee, yes, Nikon’s software does significantly reduce many lens problems when an image is opened. However, you can apply very similar fixes in Lightroom or Photoshop, so it is not anything groundbreaking. And no, Photoshop or Lightroom won’t add any CA to images – whatever you see is how your camera captured it. Adobe Camera RAW automatically fixes many lens problems with a single click. Try it – you might get better results than from Capture NX.

  2. 2
    ) Peter
    November 10, 2011 at 12:00 pm

    Good stuff.

    Very good explanations and great illustrations.

    However, good composition usually wins the day even with technical problems!

    • November 13, 2011 at 10:13 pm

      Thank you Peter! I agree, a good shot would make all lens issues appear insignificant :) I am writing about optics, because people ask these questions from me, especially when I publish lens reviews.

  3. 5
    ) Krum
    November 16, 2011 at 2:06 pm

    Very interesting. This answers a lot of questions.

  4. 6
    ) Andrew
    February 17, 2012 at 4:04 am

    Hi could you please explain what stopping down the lens is and how it helps to reduce axial chromatic aberration. Also why it can’t be used to reduce transverse chromatic aberration. Thanks very much. Andrew

  5. 7
    ) Gihan Said
    March 4, 2012 at 3:51 am

    Very interesting,thank you.

  6. 8
    ) adjoa
    April 23, 2012 at 12:19 pm

    thanks for the info.kindly explain stopping down a lens?

  7. 9
    ) Aaron
    April 28, 2012 at 10:10 am

    Almost all lenses reviewed on http://www.photozone.de have CA and distortion problems including the most expensive lenses. Then how to choose a lens and what’s the difference between expensive and cheaper lenses except for sharpness?

  8. 10
    ) Luis Rios
    July 7, 2012 at 8:55 am

    Nasim,

    Thanks for the clear explanation. I have just bought the 85mm 1.4g, in spite of the many people who have complained about its problem with CA. My rationale is that I want the absolute best portrait lens, and hope to fix CA in Capture NX/ Lightroom.

    Do you think digital corrections can overcome the 1.4G ‘ s shortcomings?

  9. 11
    ) Rick
    August 8, 2012 at 2:08 pm

    Stopping down a lens to answer someone just means changing to a numerically higher f-stop. At “wide open” using the Nikon 85 1.4 example lens described above, the maximum aperature is f1.4. Stopping down means taking the f-stop to 2.8, 4.0, 8.0. etc. which can improve or eliminate many things resolved at it’s maximum aperature. Generally speaking, a lens is also sharper stopped down a few stops instead of wide open. A lens with a maximum aperature of f1.4 is also called a “fast lens” vs a lens with it’s maximum of f4.0 or f5.6, which are generally referred to as slow lenses. Hope that helps.

    • 12
      ) Pascal
      September 2, 2012 at 1:12 am

      Thanks Nick for that clarification.
      One question remains then: what is the point of buying an expensive fast lens if you need to stop it down to get good performance?

      • 14
        ) guest
        March 6, 2013 at 8:58 pm

        Low light performance. It would allow you use to use either a lower ISO or faster shutter speed, or both. The ability to use a faster shutter speed is why a larger aperture lens is commonly referred to as a “faster” lens.

  10. October 12, 2012 at 7:30 am

    Thank you very much for this in-depth clarification

  11. April 9, 2013 at 12:35 pm

    I was wondering I anyone had an idea (or has done a comparison in the past) with the advantages/disadvantages of enabling in-camera distortion correction. Does Nikon’s algorithm produce better results with distortion/CA than LR and ACR?

  12. January 9, 2014 at 4:41 pm

    I’m doing a book on binoculars and would like permission to use some of your graphics. Of course I will give attribution.

    Cheers.

    Bill

    William J. Cook, Chief Opticalman, USNR-Ret.

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