Removing purple fringing

The joys of digital photography. You take a wonderful photo and preview it on your camera’s LCD screen just to be sure. And, yes, it looks great. When you transfer the image to your computer, you notice that your camera has an unpleasant surprise — like a cheap 1980s’ haircut, certain parts of your image are now displaying bright purple highlights. Welcome to the world of purple fringing.

You probably won’t even notice the problem of purple fringing until one day it randomly strikes. The aberration is not always purple — it can stretch into magenta and other colours, but purple is frequently reported. The issue is even more confusing since the severity and colour tint will vary between camera models and conditions.

There are many technical reasons — and some disagreement — as to why purple fringing occurs in digital cameras. These factors include the lens, focus, leakage between pixels, and more. Most of the problems are a result of the camera’s design and there is little that can be done to change these factors. It is possible, however, to understand the shooting conditions that are likely to cause problems. The most common cause of purple fringing is low light with high-contrast boundary areas in an image. For example, the picture on this page was taken in the late afternoon on a cloudy day (low light). The palm fronds are dark against a white background that is also relatively bright (a high-contrast boundary). Even under these conditions, the degree of purple fringing with this particular photograph using a new Kodak camera was a surprise.

Avoiding the problem will depend on your camera, but the more successful techniques include using a higher aperture (f4 and above) and trying different zoom settings.

Removing the purple fringing

Despite your best efforts to avoid the problem, some photos simply can’t be retaken, or you may find that banishing the purple intrusions is impossible. The good news is that it is remarkably simple to remedy the problem and, in some cases, make it disappear altogether. If you are not con­vinced, compare the before and after images .

The nature of purple fringing makes it amazingly easy to remove. Its colour range is fairly narrow and, as mentioned above, in most cases it occurs at boundaries between black and white. You could try using most of the built-in auto-adjust and ‘fix photo’ features in your graphics program, but these tend to make things worse.

There is a faster and better way — the Hue/Saturation/Lightness (HSL) tool. For details see the May 2003 Graphics column (available online here), which covered some of the hidden tricks of this under-used feature.

Instructions for using HSL to remove purple fringing will vary slightly between programs, but they all follow the same basic process. Start by loading up your affected image. Now go to the HSL tool. In Photoshop, select Image-Adjustments-Hue/Saturation… In Photoshop Elements 2, select Enhance-Adjust Color-Hue Saturation… and in Paint Shop Pro 8 Adjust-Hue & Saturation-Hue/Saturation/Lightness.

Here is the important step: click the blue channel. Do not adjust the master channel. Now adjust the saturation slider to around 80 per cent (you can fine-tune this later).

If you are using Paint Shop Pro 8, move the colour wheel slowly around and watch the preview window. When you hit the right spot, the purple will vanish. Fine-tune the slider for maximum effect and click OK. If there are still traces of purple, repeat the process until you find the other tone.

The Adobe products take guesswork out of the HSL adjustment step. When the blue channel is selected, the eye dropper tool will become available. Click the eyedropper on the left and use it to sample the purple fringe in your image. You can use the colour slider at the bottom to make finer adjustments (tip: you can also use the + and - keys to zoom in and out of the image while the HSL window is still open).

Finally, you may find the image needs some minor tinkering and will probably appear a little washed out. Making fine adjustments to the Brightness/Contrast or Levels settings will correct these problems.

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Scott Mendham

PC World

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