What is purple fringing, also called chromatic aberration?

Purple fringing on a hooded merganser female
Purple fringing is a term you may have heard of, but one that few actually understand.  It is a form of distortion which occurs in lenses and it reduces image quality significantly, especially in when in severe amounts.  It is caused by the spectral breakup of light passing through the glass elements of a lens.  Light, which is composed of different wavelengths of electromagnetic radiation, is bent when passing through any medium change at an angle (not 90°), in this case from air to glass or visa versa.

The problem is that each wavelength of light bends at different rates, which is the effect you see when white light goes through a prism.  In a camera lens the same effect is in place, which causes purple fringing as the purple (violet actually) light just starts to show its edges in the image.  You can just see the effect in image 2 above which is a little softer than would be liked.  As the area gets more and more enlarged (3 and 4) you can see greater evidence of its presence.  Image 5 clearly shows the colours associated with chromatic aberration.

In order to produce images that do not suffer from this annoying property of light, lens manufacturers have had to employ a variety of means to achieve that goal.  The upside is that we get better lenses, the downside is that they are pricey.  LD (low dispersion) and SLD (super low dispersion) glass elements are used in these high quality lenses to reduce that "dispersion", which results in purple fringing.  To further improvement, different lens coatings help.

You can facilitate better photos by using UV filters which remove some of the short wavelengths caused by purple fringing and you can shoot at smaller apertures other than wide open.  Apart from this there is another fix which can be applied "in post", referring to corrections done using programs such as Photoshop.

Image 6 above is a utility panel available in Photoshop CS that allows you to manually correct chromatic aberration.  There is an automatic version as well, but I wanted to show you this one.  In the image, I moved the "Purple Amount" slider to the right to a factor of 8 and it cleaned up the image nicely.  The final result can be seen in image 1, which is the fixed form.  The overall sharpness and quality of the shot improved significantly.

Thanks for reading.   www.ericspix.com   Eric Svendsen

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