Photoshop: Remove Purple Fringing
A simple process using Adobe PhotoShop is all that's needed to remove purple fringing from an image. While it is best to minimize or avoid purple purple fringing to start with by using an Ultraviolet (UV) filter when you're taking your photo, the problem can't always be avoided. In fact, due to the difference in sensitivity to ultraviolet light between film and digital camera sensors (film is more sensitive), a UV filter may not need to be used with a digital camera.
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 colors, but purple is most frequently reported. The most common cause of purple fringing is low light with high-contrast boundary areas in an image. The issue is even more confusing since the severity and color tint will vary between camera models and conditions and whether the photo was taken with film or a digital camera.
The nature of purple fringing makes it amazingly simple to remove. The color range within the fringe is fairly narrow and, as mentioned above, in most cases it occurs at boundaries between dark and bright areas of the image. You could try using the Auto-Levels or Auto-Color functions, but these tend to make things worse.
There is a faster and better way — the Hue/Saturation/Lightness (HSL) tool. PhotoShop isn't the only software package that provides a HSL tool. Different packages (e.g. Adobe Photoshop Elements, Paint Shop Pro) will have different instruction for using the tool. Since PhotoShop is what I use, I'll detail its use, but other packages should follow a similar process.
Photoshop's Hue-Saturation dialog
Blue channel selected, saturation reduced by -75
Start by loading up the affected image. Now select Image/Adjustments/Hue-Saturation. [In Photoshop Elements 2, select Enhance/Adjust Color/Hue-Saturation; in Paint Shop Pro 8 select Adjust/Hue & Saturation/Hue-Saturation-Lightness].
Ok, here's the important step: click the blue channel. Do not adjust the master channel. PhotoShop takes the guesswork out of the HSL adjustment step. When the blue channel is selected, the eye dropper tool on the HSL dialog will become available. Click the leftmost eyedropper and use it to sample the purple fringe in your image. Then reduce the saturation; somewhere between -70 and -80 is usually about right, but you'll need to do this by eye as every image is different. You can use the color slider at the bottom (between the two color bars) to make finer adjustments.
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.
To give you an example of the results this simple process can achieve, here's a before and after image:
Image as taken by the camera showing strong purple fringing
Image after purple fringing removed and adjusted for brightness and contrast
PhotoShop: Straighten The Horizon Videos:
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