Chromatic aberration is a type of distortion where the lens fails to focus different colors of light at the same point. In photography, chromatic aberration is also known as “fringing” or “color fringing”.
Why it happens
Different colors of light travel at different speeds (wavelengths) and when they pass through the lens they can get refracted at slightly different angles. When this happens, high contrasting objects can have a blurry colored edge around them. This “fringing” can be red, green, blue, purple, or magenta in color.
In reality, there are no “perfect” lenses. If there were a perfect lens, it would focus all of the different wavelengths of light into a single focal point. Below is an example of a perfect lens with no chromatic aberration.
The red, green, and blue light are all refracted at the same angle and meet at the same focal point.
Types of chromatic aberration
In real lenses, light is not refracted perfectly. Depending on the design of a particular lens, it can display one or both types of chromatic aberration.
Longitudinal chromatic aberration
Longitudinal aberration (a.k.a. “axial color” or “bokeh fringing”) occurs when a lens cannot focus different colors of light at the same point on the optical axis. These aberrations show up as fringing around objects and can appear anywhere in the image.
In the image above, the green wavelengths are in sharp focus where the focal plane and optical axis meet. The blue and red wavelengths are both focused at different points along the optical axis creating the aberration.
These aberrations are the most often present in prime lenses with fast apertures. The effects are hard to fix in post processing, but can be greatly reduced by stopping the lens down.
Lateral chromatic aberration
Lateral (a.k.a. transverse) chromatic aberration occurs when a lens focus different wavelengths of light at different points along the focal plane. Lateral chromatic aberrations are only visible in the corners of an image where there is high contrast. Unlike longitudinal aberrations, it can’t be corrected by stopping a lens down. Most often, you’ll see lateral aberrations in wide-angle and fisheye lenses.
In the image above, the wavelengths are all out of focus. Instead of being spread across the optical axis, they are all focused along the focal plane.
Correcting chromatic aberration
If you still have lateral chromatic aberration after reducing the longitudinal chromatic aberrations by stopping the lens down, they can be easily corrected using software. Lightroom and Adobe Camera Raw both have simple lens correction tools. In most situations, chromatic aberration can be eliminated with a single click.
In Lightroom’s develop module, the lens corrections panel is on the right side of the window. Open the panel and check the box to Remove chromatic aberration.
The process is basically identical in Adobe Camera Raw. The sixth tab in Camera Raw is the Lens Corrections tab. The image below has the tab highlighted. Inside the corrections tab, check the box to remove chromatic aberration.
In order to reduce the effect of chromatic aberration as much as possible, manufacturers are constantly attempting to make better lenses. Some lenses use special systems called achromatic or apochromatic lenses to reduce the effects of chromatic aberration.
In this type of setup, two or three glass elements with different amounts of dispersion are mounted together. The elements work together to bring all the wavelengths of light to a single focal point. Even the best apochromatic lenses can’t completely eliminate chromatic aberration, but they do greatly reduce its visible effects.
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To continue reading on the same topic, here’s the next article on spherical aberration.