Chromatic aberrations, often abbreviated to CA, is lines of color, typically along high contrast areas in your image. These lines do not reflect what the lens actually saw – it is created in the lens as the light travels through. It is also known as purple fringing or color fringing. So your lens simply adds a line of color along a high contrast area.
In the image above shot with the Nikkor 28-105mm (formerly a kit lens), the CA is really bad – notice the vertical line that follows the left hand side of the stem, but also all the leaves with white background suffer from serious CA – they are almost more purple than green!
The reason for this is a fault in the lens, where it does not manage to align the different wavelengths of light correct. The reason can be the lens design, slight movement of the glass over years (wear and tear simply) or a combination of the two.
If you notice CA while shooting, you can try to stop down the lens a bit, i.e. go to a higher f-stop number. CA is known to be worse at wide apertures, so this may help you reduce the problem.
Your post processing software can remove some parts of the CA, but not necessarily all. Lightroom has different sliders that you can try to use, if the standard checkbox “remove CA” does not work. I find that in many cases it works, but there are still a few images where I have not been able to remove the CA. If the CA is too bad and it cannot be removed in post, the only option left is to convert the image to a B&W image, as the CA is reduced to a slight blur in the image.
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