Chromatic Aberration

There are several different colour problems that affect lenses but the one we are interested in here is known as chromatic aberration and commonly manifests itself in pictures as coloured fringes with red and green edges adjacent to opposite sides of a narrow white area. For this reason, chromatic aberration is sometimes known as the “Italian flag” defect.

Ray diagram showing the origins of chromatic aberration.

The formation of chromatic aberration fringes is relatively easy to explain. It can be traced back to the fact that light is bent (refracted) when it crosses the boundary between two different media and that the degree of bending varies with the colour (wavelength) of the light. These two facts are sufficient to explain the occurrence of chromatic aberration. We have also seen that for an image to be formed, the light must pass through a single point (at the focal length of the lens). It happens that the focal length also varies with the colour of the light - and increases with increasing focal length.
The result of all of this, and the cause of chromatic aberration, is that different colour images are formed at slightly different locations. This is particularly problematic for white objects, which reflect light of every colour and for which there will therefore be a series of different colour, and differently located, images.
In other words, an image of a white object is actually a red image of that object superimposed on an orange image and a yellow image and a green image and a blue image and an indigo image and a violet image - but with an infinite number of different colour hues.
We can easily observe how a multitude of colours combine to give the visual appearance of white using a primary-school science experiment. If a disc that is divided into multiple colour segments is spun quickly then at a sufficient speed all of the colours blur together and the wheel appears to be white. Similarly, the famous Isaac Newton experiment with a prism showed that a beam of white light is split into multiple colours when the light passes through a glass prism (and can be combined back to white using a second prism).
Returning to our lens problem, it is obvious that any image that is formed closer to the lens will be smaller than those that are formed further away. Bearing in mind that blue light bends most, the blue image will be formed closest to the lens and will be smaller than the white image, creating a blue fringe on the inside edge of objects. Similarly, red light is bent least and will form an image further away, making the image larger and meaning that red fringes tend to be seen around the outside of white objects.
In the context of chromatic aberration, "outside" means closest to the edge of the frame and "inside" means closest to the straight-though direction (the optical axis).
To overcome this particular colour problem we should use glasses that generate the smallest difference between the amounts the red, green and blue light are refracted. This difference is known as dispersion and lenses that minimise chromatic aberration often use elements made from "low dispersion" glasses.
Another way to reduce overall chromatic aberration is to combine positive and negative lenses. In other words, use one lens to focus the light and another to defocus it but not to the same extent, leaving a residual focussing effect while at the same time cancelling-out the dispersion to leave a corrected (unfringed) image.
An even simpler trick, which is used in microscopy but wouldn't suit real-world photography, is to filter the incoming light so as to remove the extreme red and blue ends of the spectrum, leaving only yellow-green light. This gives a visually bright image and also improves sharpness by removing the additional wavelengths of light that would otherwise create fringes around white features within the picture. (Having said that this trick cannot be used in real-world photography, it was previously employed in b&w film-based photography but does not have the same effect in digital images owing to the inherent colour-sensitivity of the sensor itself.)
So when buying a new lens, remember to look for chromatic aberration by photographing a scene that has small white areas, such as white railings, and look fro fringes inside (closer to the lens centre) and outside (closer to the frame edges) of the white areas. If this is a visible effect then you would be well advised to steer clear of that particular lens because chromatic aberration is a difficult-to-correct image defect.

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