Alley in Pisa, Italy

by Lars Kotthoff

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I've taken this picture with a Nikon D3200 using the 50mm 1.8 lens. It contains bulbs hanging from lines stretched across two structures with a building and some sky in the background.

At the top of the image, the black lines have a purple tint to them. It seems as if this is happening where the background is the sky rather than the building.

Why is this happening, its this a known issue or is there something wrong with my camera or the lens?

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2 Answers 2

up vote 8 down vote accepted

This is known as purple fringing. It is occurs because you have a region of very high contrast. It is normal that this occurs in extreme cases but better lenses show less of it.

The good news is that software like Lightroom have tools to deal with this automatically. The Lightroom tool in particular handles not just purple fringes but other colors due to a more general problem called Chromatic Aberrations.

Some digital cameras, mostly from Panasonic and Fuji, handle it completely internally without user intervention or the option to disable it. On other models you can enable in-camera correction such as Lateral Chromatic Aberration removal but it slows down the camera significantly and reduces the buffer-depth.

Wikipedia explains it well and even has some suggestions to minimize it, like avoiding over-exposed areas and avoiding to shoot wide-open (wide apertures tend to show this problem more).

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Love the first line! "Why is there a purple tint?" => "This is known as purple fringing" –  Phil Jan 17 '13 at 17:25
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Purple fringing is a normal artifact caused by the different refraction indexes for the blue and red wave bands. Your case is a nightmare because it is a huge dynamic range difference on both sides of a very narrow line.

http://en.wikipedia.org/wiki/Purple_fringing

You have to correct for this before resizing your image, and it works best before applying bayer demosaicking, which means it works best when applied in the raw file rendering.

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