What is chromatic aberration

Chromatic aberration occurs when light breaks in a lens: colors shift towards each other. The most famous example of refraction is probably a rainbow, where the rain drops shift the colors of the sun towards each other.

In practice, we mostly see chromatic aberration in the farthest corners of recordings with sharp transitions from dark to light. How do you recognize chromatic aberration? The picture on the right shows an extreme example of chromatic aberration, where, next to the branches, ugly magenta and blue spots come into existence.


Red and blue spots

Lateral chromatic aberration 

When white light falls on the corners of a lens, the colors shift towards each other. In the picture on the right, stemming from the explanation of chromatic aberration on Wikipedia, you see how blue shifts relative to red. This is the explanation of the red and blue spots (cyan and magenta spots) on the image of tree branches against a white sky. The branches of the trees show a red border on the left and a blue border on the right. This is called “Lateral chromatic aberration” (often referred to as “Red/Cyan Fringe”).

220px-Chromatic aberration convex.svg
A more detailed explanation of chromatic aberration can be found here. For photographers, it is sufficient to recognize the phenomenon and correct it, as this form of chromatic aberration can be controlled. Photoshop and Lightroom offer the possibility to correct for chromatic aberration.

Green and Red spots = Color Bokeh

Secondary longitudinal chromatic aberration

This is a rare form of chromatic aberration. It gives a green color cast of blurred subjects closer than the point in focus and a purple color when subjects are behind the point in focus. All lenses can suffer from this phenomenon, but in general, fast lenses (with a larger aperture than aperture 2.8) are more sensitive to color bokeh. In the macro recording on the right, the antennae on the head of a butterfly display a green color cast and a magenta color cast. Click here for another case study of secondary longitudinal chromatic aberration, which we encountered during our tests. This form of chromatic aberration can be seen on the whole picture and cannot be corrected with chromatic aberration correction in Photoshop and Lightroom afterwards. Aperture does help against this phenomenon.


Purple spots = Blooming

Usually no chromatic aberration

There is another form of chromatic aberration, which can be recognized with purple spots. But a phenomenon that we encounter more often in practice – and is often mistaken for chromatic aberration – is blooming. These are purple spots caused by overexposure of individual pixels on the sensor. The pixel flows as it were, as with a full bucket of water, causing purple (red + blue) spots in the neighboring pixels. CCD sensors (which are in most compact cameras) are more sensitive to blooming than CMOS sensors (which are used in most cameras with interchangeable lenses). You encounter this phenomenon therefore more often in photographs taken with a compact camera than in photos taken with an SLR.


More on Chromatic aberration:


Secondary Longitudinal Chromatic Aberration Color Bokeh


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