This is just a short piece to let you know of a very useful Lightroom tip that can help you with getting rid of purple-green fringing (Chromatic Aberration) in your images.
Lightroom has a powerful CA removal tool in the Lens Corrections | Color panel, and I strongly suggest that you have the Remove Chromatic Aberration box checked by default.
This does an excellent job of removing red-green and blue-yellow fringing in images (something I get a lot of at the wide end of my 17-40mm and 24-105mm lenses), but some purple and green fringing can still remain.
To target and reduce purple/green fringing Lightroom allows you to either select the offending hue using an eyedropper tool, or to manually select a range of hues using split sliders.
Now the eyedropper is a useful tool, but I often struggle to get it to recognise and ‘lock on’ to a purple/green fringe no matter how pronounced the fringing might seem to me. So, manually fiddling with the split sliders to identify the correct hues is often the only way I can tackle the fringing.
So here’s my quick tip (at last!): set the Amount to, say, 3 so that defringing is active, click on the bar between the two end sliders of either Purple or Green Hue and drag the sliders left and right along the range of hues (as shown in the animation below). Keep an eye on the image as you are doing this, and you should see clearly which part of the range of hues is most effective. You can then adjust the split sliders to specify a narrower range of hues, and again slide the range left and right to see which amounts are most effective. I often find this a much quicker and more effective way of targeting the fringing hues than using the eyedropper.
I have an animated gif file here to illustrate the point. Although some of the colour has been lost from the image in creating the gif, you can see how the fringing changes as the two sliders are moved along the Purple Hue range.
One last word on Defringing: when I am successful in using the eyedropper to select a hue to defringe, I find that the Amount value which is automatically set is often excessive. If I drop the Amount to a much lower figure the fringing is still effectively dealt with.
For a clear and comprehensive illustrated outline of the colour fringe tool in LR4 please read this article by Eric Chan in the Adobe blogs: http://blogs.adobe.com/lightroomjournal/2012/04/new-color-fringe-correction-controls.html