Chromatic Aberration (commonly known as “CA”) is something that can occur with lenses that appear as bright colors which weren’t present in the scene at the time the picture was taken. Believe it or not, chromatic aberration is present in nature too. Have you ever seen a rainbow? We see rainbows because of the prismatic effect of light when it passes through droplets of water in the air. While camera lenses are designed to focus all the wavelengths of light it’s difficult to make that happen perfectly across the entire image frame.
CA doesn’t affect most of the image.
Technically speaking, if a lens has Chromatic Aberration it is always there in the image. However, since CA is comprised of two-color opposites such as magenta and green the two colors in CA tend to cancel each other out within low contrast areas so CA doesn’t generally affect how most of the image looks. Wherever there are high contrast lines in your images, such as silhouettes of trees or other objects against a bright sky, or even the high contrast borders created by a black and white tuxedo, CA can appear due to the relative difference in brightness. As we know, white is bright, and black is dark. White means more light is coming through. So, when the bright white areas of a dress shirt are projected by the lens right next to the edge of the black suit, there is more misfocused light coming from the white area, which causes the CA to show up long the dark areas where the suit overlaps the white dress shirt.
How to identify and deal with Chromatic Aberration in your photos.
While you might think that all lenses, especially expensive ones, would be corrected for CA (Chromatic Aberration), you would be wrong. Unfortunately, many lenses will show some chromatic aberration to varying degrees. If you are unlucky and get a “bad copy” of a lens, it may even be so annoyingly obvious that you’ll need to return the lens.
Many professionals like me try to purchase lenses that don’t have a lot of Chromatic Aberration for one simple reason. We don’t want to have to look for it and fix it in every photo we take. While it is common to use Lightroom and Photoshop to remove CA, it is not always as easy as clicking the “remove CA” button in Lightroom. CA is actually one of the worst and most annoying aberrations even if it is commonly believed that it is easily removed. The reality is that it has a nasty habit of sneaking into images even when we think they’ve been “fixed” by Lightroom.
Purple Fringing
Lightroom is really bad at removing certain types of CA, particularly what is called “purple fringing”. Purple fringing is not the same as the “CA” that Lightroom can automatically remove in images. True CA has a positive and a negative aspect. That is to say, true CA has TWO colors, typically these colors will be spectral opposites. Purple fringing does not have this spectral dualism, instead, purple fringing appears only as purple on its own. This makes it a lot harder for Lightroom to detect and remove the purple fringing in an image because it can’t be sure that the purple isn’t supposed to be in the image in the first place.
Technically speaking Purple Fringing is a misfocusing of certain wavelengths by the lens similar to CA but is actually a kind of lens flare known as blooming. While purple fringing often appears around high contrast edges, it can appear anywhere in the frame with equal intensity and tends to appear most on the plane of focus. Specifically, purple fringing tends to appear around light sources or strong specular highlights. It will often appear off to one side of a strong highlight or completely surrounding strong highlights, which is different from the behavior of CA. It can be present at the same time as CA causing discolorations of the CA.
Purple fringing is one of the worst types of CA to have in a lens because it often appears on top of any existing Chromatic Aberrations, particularly Transverse Chromatic Aberration. This causes the CA to appear even stronger than it really is greatly exacerbating the distracting effect of bright purple/blue lines around high contrast areas.
Removing Purple fringing can be a huge challenge as Lightroom’s tools for removing CA can have unintended consequences and create a patchy look to the image if overused. The best option for removing Purple Fringing is usually to manually Photoshop it out of the image.
Axial Chromatic Aberration
While CA on the plane of focus is super annoying and ugly, many lenses attempt to correct CA on the plane of focus but allow CA further away from the plane of focus. CA that occurs away from the plane of focus is called AXIAL CHROMATIC ABERRATION. The good thing about Axial Chromatic Aberration is that it is usually reduced by stopping down. The bad thing about it is that if you like blurred out backgrounds then Axial CA might be a major issue for you. Unfortunately, due to the blurred nature of Axial CA in backgrounds it is very difficult for Lightroom to remove. Axial CA also has two color parts, but in the case of AXIAL CA, one color such as green may appear behind the plane of focus and its spectral opposite will appear in front of the plane of focus. Depending on how you tend to frame your photos you may rarely if ever see the spectral dualism of AXIAL CA in a single image.
Axial CA is also very hard for Lightroom to identify and remove. Most of the time Lightroom will fail to remove Axial CA and often times won’t make a difference at all. Again, the best way to remove Axial Chromatic Aberration is generally to Photoshop it out of the image manually.
Transverse Chromatic Aberration
CA on the plane of focus is called Transverse CA. Transverse CA is what Lightroom is best at correcting in an image. Transverse CA usually possesses strong spectral dualism and if present, is usually more visible towards the edges of the image circle. Lightroom can often easily identify and remove transverse CA using the Camera Raw Tools in Photoshop and Lightroom.
One caveat to this is that “Remove CA” doesn’t actually remove all the Transverse CA and that is usually because there is more color there from Purple Fringing. CA should appear in equal ratios on each side of a high contrast edge. If the ratio isn’t 50/50, Lightroom won’t fully remove the CA in the image. As I mentioned above the ratio is usually thrown off by Purple Fringing which can easily be mistaken as Transverse CA, especially because it tends to appear in the exact same areas.
