When a lens fails to bring all wavelengths of color to the same focal plane, or when wavelengths of color are focused at different positions in the focal plane, it causes chromatic aberration, which is also known as “color fringing,” “purple fringing,” or “blue fringing.” When light passes through a lens, different colors travel at different speeds, resulting in chromatic aberration. As a result, images in high-contrast situations may appear blurry or have noticeable colored edges (red, green, blue, yellow, purple, or magenta) around objects.
The best focus with the “circle of least confusion” is located at the focal point of a perfect lens, as shown below:
Lenses have refractive indices that differ for each wavelength, resulting in two types of chromatic aberration: longitudinal and lateral.
1. Longitudinal Chromatic Aberration
Longitudinal Chromatic Aberration: “LoCA” or “bokeh fringing” occurs when different color wavelengths do not converge at the same point after passing through a lens, as shown in the illustration below:
The image can have fringing around objects all over it, even in the center, if the lens has a problem with Longitudinal Chromatic Aberration. Things can have haloes of different colors or even a combination of them. When using a zoom lens, stopping it down significantly reduces longitudinal chromatic aberration. LoCA is much more common with fast aperture prime lenses than slower ones.
When viewed from various distances, this example of transverse chromatic aberration can be seen:
Note how the green color at the top of the image changes to a neutral tone in the middle, and then turns purple as it gets closer to the camera at the bottom. Even high-end, expensive lenses like the Nikon 35mm f/1.4G have this type of longitudinal chromatic aberration. Post-processing can significantly reduce the amount of LoCA/bokeh fringing in images. For example, the “De-Fringe tool” in Lightroom 4.1 allows you to pick a fringe color that needs to be corrected by selecting an eye dropper in the “Lens Corrections” module. Using such a tool, it is possible to either eliminate or significantly reduce this type of fringing.
An additional example of green and purple fringing caused by longitudinal chromatic aberration can be found here.
With a single click in Lightroom’s “Lens Corrections” sub-module, the bottom crop was corrected. The same thing can be accomplished in Photoshop, but it necessitates a greater number of steps (if not using the Camera RAW tool).
2. Lateral Chromatic Aberration
Lateral Chromatic Aberration is also known as “transverse chromatic aberration. “Color chromatic aberration occurs when different color wavelengths focus at different positions along the same focal plane, as shown below:
While LoCA appears in the center of the image, Lateral Chromatic Aberration can only be seen in high-contrast areas near the edges of the image. Some fisheye, wide-angle, and low-quality lenses exhibit blue and purple fringing. Contrary to Longitudinal Chromatic Aberration, Lateral Chromatic Aberration can be reduced or eliminated in post-processing software rather than by stopping down the lens.
Corner CA in the Nikon 35mm f/1.8G lens is quite severe, as you can see in this crop:
Longitudinal chromatic aberrations are common, but so are lateral chromatic aberrations. Lightroom and Photoshop both have tools for fixing lateral chromatic aberration, but they don’t have anything for stopping down the lens (which reduces LoCA).
Achromatic/apochromatic optical designs and special extra-low dispersion elements are among the techniques many modern lens manufacturers use to reduce chromatic aberrations, but the problem persists on most prime and zoom lenses, and we must learn to work around it. While this may sound discouraging, the good news is that many modern DSLRs come equipped with special in-camera post-processing techniques that can significantly reduce or even eliminate lens chromatic aberrations.
3. How To Avoid Chromatic Aberration
We’ve discussed chromatic aberration, including what it is, how it occurs, and how it appears. By now, you may be feeling discouraged because you believe you’ve discovered a new “problem” with your photography.
You should know that chromatic aberration can be controlled and possibly avoided altogether. A few techniques can be used to minimize the impact of chromatic aberration on your images, so let’s look at them now.
3.1 – Shoot in RAW
To begin, I recommend shooting in RAW for a slew of reasons other than chromatic aberration. Unless you plan on doing extensive post processing on your images, there is no reason to shoot in JPEG in the first place. Only by shooting in RAW will you be able to get a good starting point for your edits.
It’s easier to experiment with post-production software, such as Adobe Lightroom or Adobe Camera Raw, when you shoot in RAW instead of JPEG. With a little practice, you’ll be able to eliminate nearly all chromatic aberration from your images.
3.2 – Avoid High-Contrast Scenes
Chromatic aberration is more noticeable in high contrast scenes because of the sensitivity to it. Of course, if the shot you have in mind necessitates working in conditions of high contrast, you won’t have much choice but to take the shot regardless. However, if you have a choice between shooting with high contrasts in the frame or not, I’d choose the latter since you won’t have to deal with the evidence of chromatic aberration later on.
3.3 – Focal Length
Chromatic aberration is most noticeable when using a zoom lens at its short and long ends of the zoom range. For example, if you have a 24-70mm lens, you’ll see more chromatic aberration at 24mm than at 70mm.
There are two ways to deal with this: either use a prime lens, which is better at handling chromatic aberration, or only use the focal lengths in between the minimum and maximum of your zoom lens. If you want, you can crop or create a panorama to match the lens’s minimum and maximum focal lengths.
3.4 – Aperture
Longitudinal chromatic aberration can be reduced by experimenting with your aperture. How? It’s simple: reduce your lens’s widest aperture by one or two stops (or more).
Shutter speed and ISO need to be lowered or raised when using small apertures, but this will help reduce chromatic aberration over time.
4. How To Test Your Lenses For Chromatic Aberration
Google “Chromatic Aberration Test Chart” and download one of the free models to see how effective your lenses are at reducing chromatic aberration. Now open it up on your PC in full screen mode with the brightness set to its highest setting. You can also print it out and use it as a reference to take a few test shots with different lenses.
When you open the photo in Lightroom and zoom in on the dots, you’ll see just how much chromatic aberration is affecting the image.
It really is as simple as that!
By now, you should have a much better understanding of chromatic aberration, including what it is and how to avoid it during the shooting process as well as in the editing phase. Chromatic aberration is a bothersome issue, but don’t freak out if you notice it in some of your photos. Controlling them in the field is simple, and removing them in Lightroom is even simpler.