Have you ever heard the expression ‘the devil is in the details?’ In photography, the devil is called Chromatic Aberration. CA to its friends.
But what is chromatic aberration? It usually manifests itself in the form of purple/red/blue/cyan/green fringes alongside highly contrasted edges. It’s easily spotted if you check your photos at high magnification.
Top: The dawn on a beach in the Kerkyra Island (Greece). Bottom: 100% crop on the dock reveal quite a bit of chromatic aberration along the high contrasted edges.
This is probably cold comfort, but those halos are not your fault. You did nothing wrong and CA is simply the outcome of light passing through your lens.
In this article you will learn how to deal with CA by minimising it in camera and how to correct it with Adobe Lightroom in post-processing.
Don’t worry if you don’t have Lightroom. Similar CA correction methods are found in almost every editing software, including the one coming with your camera (e.g., the Canon Digital Photo Professional, DPP, editing software).
What Causes Chromatic Aberration?
An exhaustive discussion about the nature of chromatic aberration is beyond the purpose of this article and would require a solid knowledge in optics. But in laymen terms we could say that chromatic aberration consists in finding colours where they shouldn’t be.
White light does not exist and white is not a colour. It is our way, as human beings, to perceive the combination of the different visible colours making up the visible part of the electromagnetic spectrum. We only see a mere 0.0035% of the entire electromagnetic spectrum.
Every colour behaves in its own particular way when passing through a material. This is why a prism can separate those colours from each other, or “disperse” them, and we see them individually, forming the familiar rainbow.
The so-called refractive index of the material that the white light is passing through causes the dispersion.
Refraction is stronger for light of short wavelengths (blue), and less intense for light of long wavelengths (red).
And finally, different kinds of glasses cause refraction or dispersion of various intensities.
Dispersion of white light passing through a prism. Image Credit: Wikipedia.
The Different Types of Chromatic Aberration
There are two types of Chromatic Aberration: Longitudinal and Transverse CA.
Longitudinal Chromatic Aberration
Optical scheme illustrating longitudinal (or axial) chromatic aberration, LoCA (left), and how it looks like in photographs (right).
What is Longitudinal Chromatic Aberration?
Longitudinal (or axial) Chromatic Aberration, LoCA, occurs when the lens cannot focus all the different colours on its focal plane (the sensor). One or more colours are then focused either in front or behind the focal plane.
What does it look like?
LoCA appears in highly contrasted areas and it can be visible at the edges or at the centre of the frame. It presents itself as a blurred purple or green fringe either in front or behind the object in focus.
The image below shows a fair amount of chromatic aberration. It’s along all the highly contrasted edges, regardless of their position in the frame.
This image present a fair amount of chromatic aberration.
LoCA is easily identified. It changes colours when you focus in front or behind the object and it disappears when you move your lens to a narrow aperture.
LoCA changes from purple to green by changing the focus (left) and disappears when stepping down the lens (right)
From the image above, you can see that the blurred purple colour in the image taken at f/1.4 is also in front of the window edge and not just alongside it. This makes it difficult to correct in post processing.
Transverse Chromatic Aberration
Optical scheme illustrating transverse (or lateral) chromatic aberration, TCA (left), and how it looks like in photographs (right).
What is Transverse Chromatic Aberration?
Transverse (or lateral) Chromatic Aberration, TCA, occurs when the different colours are focused on the lens’ focal plane, but not all in the same point. It is more present toward the edges of the frame and is not shown in the central part of the frame.
What does it look like?
TCA appears as sharp colour fringes alongside highly contrasted edges of a dark or bright area. Edges at the opposite sides of such areas show colour fringes of complementary colours (e.g. left side green, right side red).
Stepping down your lens does not correct TCA.
This 100% crop of the opening image in this post exhibits a quite strong TCA along the poles of the dock.
How To Reduce Chromatic Aberration on Your Camera
The lens industry puts great effort into minimising chromatic aberration in their lenses.
They do this with low dispersion optical glass and optical elements. Apochromatic lenses, for example, are specifically designed to correct chromatic aberration.
Despite these efforts, a certain amount of both LoCA and TCA is, unfortunately, unavoidable.
High quality lenses exhibit much lesser chromatic aberration than:
- cheap lenses;
- fast lenses when used wide open;
- old legacy lenses;
- cheap teleconverters and wide angle converters.
In order to reduce chromatic aberration in-camera, the most effective tricks you can try are:
- avoiding highly contrasting scenes;
- accurately focusing to reduce the LoCA blur, making it look smaller;
- stepping down your lens by 1 stop or 2, i.e., by using smaller apertures, to remove LoCA;
- placing your subject in the middle of the frame to make it TCA free and crop later to regain a better composition;
- exposing for the highlights and avoiding blowing them up;
- avoiding to use the shortest and longest focal length on zoom lenses;
- varying the distance between you and the subject.
How To Correct Chromatic Aberration in Adobe Lightroom
TCA is usually easier to correct in post-processing than LoCA, but, in general, Adobe Lightroom lets you remove (or greatly reduce) both with just a few clicks.
In Adobe Lightroom, both an automatic and a fully manual correction for CA can be found in the Lens Corrections panel of the Develop Module.
Automatic CA correction (left) and defringe settings for manual CA correction.
Chromatic Aberration Automatic Correction
Nothing could be easier. Tick the box Remove Chromatic Aberration in the Lens Corrections->Profile panel and let Lightroom do its magic. Hopefully most, if not all, of the LoCA will be corrected and you can stop there.
Comparison between original image with visible TCA (top) and the autocorrected image (bottom).
CA Manual Correction
Sometimes, CA automatic correction does not remove all the CA in your image. In these cases you can go to the Lens Corrections ->Manual panel and use the eye-drop in the Defringe section to sample the CA along the affected edges.
While viewing the image at 100% sample different points along the edges showing CA. Like this, you’ll get the best global result. Repeat the procedure for both the purple/red and blue/green colours.
A fully manual way to correct CA consists of manually manipulating the amount and colour hue sliders in the defringe section of the Manual panel.
Those sliders control the width, in pixels, of the colour fringe to correct and its colour hue, respectively. They are available for both purple and green colour fringes.
You can also manipulate those sliders to further reduce, or completely remove, any residual CA that was not corrected with the automatic and/or eye drop methods.
The same image of before, but this time I manually corrected the AC.
When you manually correct your photo for the AC, do not focus only on the edges you are sampling. Check the rest of the image as well.
Finally, don’t push the correction too far, as this could negatively affect some other parts of your image.
Cranking up the hue for the purple color all the way had some negative impacts on the church in the centre of the frame, where some areas have been greyed out.
The custom chart I use to test my lenses for chromatic aberration. Right click on it to download it.
Right click on the image above to download the chart I use to test my gear against CA, then print it or display it full screen on your monitor set to its max brightness.
Now you can start systematically testing your lenses against CA and how you can minimise it in camera.
Below is the result I obtained with my Sony Rx100 Mk2, set full wide (28mm equivalent) and wide open (f/1.8), and the chart displayed on my computer monitor.
Testing the Sony RX100 Mk2 at 28mm equivalent and f/1.8 against CA. Cromatic Aberration has been maximised in camera and in post processing to make it more visible.
To be fair to Sony, the CA of this camera is not, usually, that bad. For the sake of clarity, and to make CA more visible for this article, I have intentionally maximised the CA by overexposing the scene in camera by 2 full stops. In Adobe Lightroom I have also cranked up the vibrance slider all the way up to make the CA pop.
The Sony RX100 Mk2 is affected by both TCA (automatically corrected by the software) and LoCA (manually corrected with the eye-drop method).
Despite how I have worked to maximise the amount of CA in the image, the correction tools available in Lightroom are so good that the complete removal of CA was just two clicks away (see image below).
Chromatic aberration corrected version of the previous image.
In short, if Chromatic Aberration shows up in your photos, don’t panic! After reading this article, you now know several tips and tricks to get rid of CA. You can minimise it on your camera and correct it in post-production, so that your photos turn out just the way you want them to.
If you’re new to using Lightroom to organize and post-process your photos you can learn more with our beginners’ guide.
And check out our 15 Keyboard Shortcuts to speed up your Lightroom workflow.
A note from Josh, ExpertPhotography's Photographer-In-Chief:
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