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:
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:
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:
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:
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:
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:
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.
If you are interested in reading more, below is the list of articles on other types of aberrations and issues that we have previously published on Photography Life:
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.
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.
Good stuff.
Very good explanations and great illustrations.
However, good composition usually wins the day even with technical problems!
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.
Very interesting. This answers a lot of questions.
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
Very interesting,thank you.
thanks for the info.kindly explain stopping down a lens?
Almost all lenses reviewed on 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?
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?
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.
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?
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.
Thank you very much for this in-depth clarification
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?
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.
i got all the answers i needed
Great article, thanks for the write up. But why does it occur in high contrast areas AROUND the objects? How come I don’t see it in the object itself? I take lots of tree photos, and it’s always there outlining the tree, but I never see it actually in the tree.
Patrick, because light rays at different wavelengths do not converge at the same point, as shown in illustrations. You will never see it with your eyes, but lenses do reveal them. CA depends heavily on lens design – some lenses are much better at handling CA than others.
Hi Patrick
The previous answer seems to answer something a little different.
To answer your question: It’s when there is a large contrast that the effect becomes visible. Within the object (in your example the tree) the contrast is not large enough to produce any visible effect in your image. The same amount of CA is happening within the tree, but because the difference in color intensity isn’t large, it isn’t a problem here.
why is chromatic aberration only in lenses? how come chromatic aberration does not occur in mirrors? mirrors only have spherical aberration.
Chromatic aberration in lenses is a symptom of different wavelengths refracting more or less through the medium. When light passes through any given medium other than vacuum, it slows down, and higher energy light (blue) slows down more than lower energy light (red). The path light takes through the medium also bends in accordance with the material’s index of refraction, and angles of incidence and departure.
With mirrors, you’re only dealing with angle of incidence and departure, which is always the same regardless of wavelength. Pure reflection, no index of refraction. Blue light comes off the mirror at the exact same angle as red light.
Catadioptric systems use both mirrors and refractive optics, and are used in pretty much all mirror lenses for cameras. They’re not immune to chromatic aberration.
I’d imagine because with lenses the light wavelengths are passing through a medium, a lense, bending the light, and with mirrors they are simply reflection/deflecting off a surface.
Now if we can just get video games to stop putting this “option” in. At least they let you turn it off I guess. It’s like with lens flare or god’s rays. They keep acting like in videos games your “eyes” are camera lenses instead of putting you in the shoes of a “person” with eyes. Eyes don’t have light rays or chromatic aberration.
I’m sorry but that last sentence was bullshit. Of course your eyes produce chromatic abberation.
Thank you for this information. But I see it with my eye all day long. I have an IOL (inocular lens) implant and I tried to explain to the dr what I was seeing. This helps me understand what is wrong, and explain what I’m seeing. Now to figure out how to correct it in either a contact and or glasses.
Great information. Any help is welcome