What causes purple fringe
As the title says what causes purple fringe when taking storm or mountain shots. the second part is how do I avoid it.
I recently was taking shots of a beutiful mountain range that was being hit by a pretty nasty storm. The shot is awsome until you blow it up to large format which when i saw it i was very disapointed. any informed advice would be appreciated. Camera specs in my sig
Camera: Canon 60D with Grip and IR remote.
Lens:Canon 18-200, Canon 100-400L, Canon 2xIII teleconverter
Flash: 580 Speedlight
Memory: 4x 32GB class 10 SD
Purple fringing is Chromatic Aberration (CA):
The problem is probably your 18-200 lens which has a reputation for high CA especially at the wide end of the focal length range. It seems to be a design problem with "superzoom" lenses which cannot be optimised for the whole focal length range.
CA can be removed to some extent in post processing (google "chromatic aberration removal"). It is easier to remove if you shoot Raw. I have a lens (Canon 17-85) which has quite bad CA at 17mm but Lightroom's raw processor does a very good job of removing it and I normally only see it in extreme shots, e.g. branches of a tree against a bright sky.
I'd suggest you google "fix purple fringing" instead as CA is a generic term which technically covers purple fringing, but common CA tools won't fix purple fringing, just a different type of CA.
Typically the fixes try to select the relevant colours and desaturate them.
Using filters will increase the likelihood of PF. Yet another reason to avoid them. Using a hood can help insofar as it reduces stray light entering the lens from off angles.
Any shot with high contrast can have PF, but it does depend mainly on the lens design, coatings, etc.
There are many possible causes, and without pictures to examine, we couldn't even hazard a guess.
Here's a Wikipedia article on the topic — I don't agree with it 100%, but it's a start:
Another fringing problem not mentioned there is blue fringing around foliage. This occurs when shooting against a bright blue sky, with the sky overexposed, and the foliage out of focus. The blue fringe is the true color of the sky, in the area where the overexposure is prevented by the mis-focus blur from the foliage.
Different wavelengths of light refract differently, which I'm sure has caused many a headache for lens designers. When red and blue wavelengths are out of focus in relation to green wavelengths, they combine to form a magenta/purple color.
Some good information on CA and fringing: