Again ... for those who can't afford better equipment and might be affected by this fringe thing. I have been.
Fringe, sometimes called "purple fringe" (there are other colours), is caused by the lenses' optics. It's a cromatic aberration where lenses decompose light on its components and project them slightly seperated in the sensor (or film).
It is most noticed on hard edges, transitions of high contrast, not always brightness wise, but also in places of high contrast changes in each light component.
The effect is also more prone to happen when photographing in high mountains where UV rays are stronger and light enters the lenses in weird ways/directions.
In terms of hardware you can get more expensive lenses with low dispersion elements that greatly reduce fringe. A good UV filter is also a good thing.
In terms of shooting use your lens hood and/or have the sun on your back as much as possible.
Most raw processing software has options to eliminate or reduce fringe, this includes of course LightRoom, Camera Raw and even inside Photoshop you can find a tool to eliminate fringe issues. But, once again, this is about dirty tricks and not about tuturials on those tools :)
But, when shooting in higher mountains, with lots of hard sun, with bright clear rocks everywhere reflecting everything ... fringe can be just overwhelming when using up to an average lens! I've been in such a situation late July, photographing in National Park of Gerês, north of Portugal. And many of my images were uterly ruined by this! But in all my efforts I developed a couple of ways of still making some images usable.
My biggest problem was that sometimes fringing was not uniform all over the image! That just throws away every tool I mentioned before! To correct in one side I ruined the other side!
In one particular image, that in spite of all my efforts ended up rejected ;), I had blue fringe in the middle right, yellow in the middle left, purple at the top and green at the bottom! This picture of a more less neutral rock (strangely shaped like a man's head wearing a hat) looked more like a christmas tree with so many colours!
If light components are seperated then we can fix it by displacing each channel the right amount to get it in synch with the other channels. In a simple example, if you have blue fringe to the right, enter the channels folder of photoshop, select blue channel and move it one pixel to the left. One, tops 2 pixels should be enough. If not ... throw your lenses in the garbage! ;)
But if it was that simple those other tools could have taken care of it!
If fringe is not uniform you can at least assume that is changes uniformly along an axis. That image I spoke of before, the blue fringe become less noticeable to the center and then it changed to yellow on the left side. I could then select the half right part of the blue channel and shrink it one pixel towards the center. It really works! I've saved a couple of images using this kind of trick.
I just need to additionaly crop one column of pixels on the right edge (because there's no blue any more in there).
But for this particular rock head I had to do the same for the yellow (that is abscense of blue in RGB), and then vertically for green and for red ... and the whole image is somehow clean of fringe, but looks like crap! :-|
Fringe is created mostly on hard contrast edges, those spots are corrected, but on every other places of smooth transitions, visual information of each channel was just moved away from its place and the image gets fuzzy, with details mixed up! If you pay close attention, maybe zooming to 200% or 300%, even those tools I mentioned in the beginning, under uniform fringing, cause some of this fuzziness and images are no longer so sharp. If messing around with every channel ... well, results were predictable after all!
Going around fringe
If the surfaces where fringe exist are mostly of one colour, like the grey stone I was talking about, there's another trick, dissimulate the fringe. Keep every channel in place, pick the dominant colour, and then just paint over the fringe pixels! Use some kind of overlay technique to avoid destroying as much detail as you can, you just need to change the colours of the pixels, not contrast, not detail. In my case I just used a brush in saturation mode and dessaturated fringe pixels.
Of course it was not perfect, but after using these two tricks I decided the image became usable and clean enough to submit. It was not, but in less complex situations I was able to recover very nice pictures, specially landscapes (watch for those skylines when looking for fringe)
Sharpen brings up fringe issues, so does increasing contrast and saturation. Be carefull, maybe your lens is not that bad and you're just overdoing it ;)
Reducing image size may also greatly reduce fringe visibility, and, if done right, for more than the reduction itself. Just open your raw, set your sharpen, saturation, vibrance and contrast to zero, import it that way to your image editing software, reduce 50% and work from there.
But I have a 6Mpxl sensor ... if I reduce 50% I can only suply icon size files and DT doesn't have a market for those ;) But I've also used this in large panoramas made of several images.
So now, if you have images with bad fringe problems, what are you going to to?
For me ... what is done is done, but the photographic results of that trip were so disapointing on that particular subject that I finally decided to invest the money and have just bought a prime (maybe sub-prime) lenses to replace the one that came with my camera. I realy hope I don't need to learn/develop more tricks like these from now on!
The line of photos above have been shot with the new lenses, I've thrown at it pretty hard situations and I can tell you it still is not perfect on this fringe issue, but it is so much reduced that is realy negligeable, a completly new world, and not just for fringe issues :)
P.S. I need a garbage can ;)