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Tuesday, 9 October 2012

Correcting colour fringing (short tutorial)

Since I took up photography three or four years ago I have started to notice technical imperfections in my photographs much more than when I first began. I'm sure this happens to all photographers as they become more experienced. It was just over a year ago that I first became aware that certain pictures of mine had an unwanted fringe of colour along the edges of particular objects. A casual viewer would probably not have noticed; indeed, I have one or two affected pictures online that have attracted positive comments but no-one has mentioned the colour fringing.

The technical term for this is chromatic aberration, and it occurs when a lens fails to focus all parts of the colour spectrum to the same convergence point (more thorough technical explanations can be found online). Needless to say, cheaper lenses are more likely to result in chromatic aberration than their more expensive counterparts. In my own photographs I have noticed that the effect occurs mainly when I am taking sunset or sunrise pictures, and the edges of buildings seem to be particularly susceptible.

When I was searching online for a post-processing solution, most of the search results (at least those near the top of the list) seemed to refer to the use of Photoshop, which I don't possess. However, it turns out that Apple's Aperture software, through which all of my photos are processed,  has the solution within it.

Here is a magnified section of a recent photograph, showing a portion of the London skyline.  You can see a green edge down the left side of the buildings and a red edge down the right hand side. This screenshot also displays the Inspector in Aperture, with two adjustable sliders in the menu option for Chromatic Aberration. At this point they are both set to zero (no adjustment).

In this next screenshot, the slider for the Red/Cyan adjustment has been shifted to the left. As you can see the colour fringing around the buildings has largely disappeared, and certainly is not visible when the picture is seen without magnification.

It is also possible to use "brushes" to brush away colour fringing in specific regions of a photograph. Of course, if I had read the user manual for Aperture in the first place then I would have known about this!

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