A PhotoShop plugin, developed using FilterMeister, that reduces CA-like blue fringes on the Sony DSC-F828 and probably other digicams as well.
This is a work in progress, but already works extremely well on many landscape images, so I'm sharing it now. See also the companion plugin PFree for reducing purple fringes around overexposed highlights.
There are a zillion theories about purple fringing in general, and particularly on the Sony 828. For my personal musings look here. But generally you don't need to have an opinion for this to work. It does help to have some understanding of the causes in order to avoid PF in the first place (avoid shooting wide open in high contrast scenes, and avoid overexposing highlights, particularly specular reflections and bright sky behind dark objects).
It seems to me that there are actually two causes of fringing, one caused by chromatic aberrations, and one by a sensor effect (scattering of light on the sensor surface plus electronic effects?). The CA type is geometrically predictable and as far as I can tell is mainly a problem in the blue channel at wide angle in the corners of the frame. The sensor type can happen anywhere in the frame around overexposed highlights, affects the red and blue channels (and to a lesser extent green) and is thus more troublesome.This plugin only addresses the CA type.
The basic approach is to subtract a scaled copy of the blue channel (or another if you want) from the image, eliminating the radially offset ghost image that creates the perception of fringe. This is slightly different from normal CA correction, where you simply rescale a channel to change its magnification.
Drop the filter (download below), which is an .8bf file, into your PhotoShop filters directory, usually something like c:\program files\Adobe\Photoshop\PlugIns\Filters. Then launch PhotoShop. I've tested this with Photoshop 5.0LE and CS, and PS Elements 2.0. It's not demanding and should work with other versions as well.
If you're using other software that accepts PhotoShop compatible plugins installation should be similar, but I don't have any examples to try. CAfree might also run standalone with PluginCommander but I haven't tried that either.
CAfree opens to a control dialog with a preview at left. You can drag on the preview to move around your image or selection. You can zoom using the +/- buttons below the image or the top slider. The preview is by default zoomed to 100%; you can vary the zoom for a general impression, but the controls aren't fully scale-aware, so you should fine tune your settings only at 100%.
This is not exactly commercial-grade software, but unlike it's companion PFree it's very simple. Fortunately, the defaults work fairly well in many circumstances, and the preview is quick so it's easy to experiment. Generally I find it better to err on the side of undercorrection, removing just enough to eliminate the obvious impression of blue.
The filter processes the whole image, but doesn't do much to the center and to areas of low contrast. You may notice slight shifts in color balance, and perhaps a slight overall improvement in the perception of sharpness. It only touches one channel at a time (normally blue). If you want to correct another channel, too, just run it again.
Because the filter uses the degraded image to estimate the original, uncorrupted image, if you're going to use the companion PFree to fix fringing on overexposed highlights, do that first, then run CAfree.
Most importantly, the filter expects that the optical axis is at the geometric center of the image, so you should only run the filter on a complete image, or a centered selection, or a crop that's padded with whitespace to place it correctly where it would have been in an original.
Running the filter on an 8mpix image takes under 10 seconds on a 2.4GHz P4.
The following image is generally moderate in contrast, with no overexposed highlights, shot at 47mm (almost 200mm 35equiv.) on a Sony 828, at F2.8.
Looking closely at the bottom right corner, around the light alder branch, there's a bluish fringe above and to the left (toward the center of the frame):
This can be corrected using CAfree, with the default settings, except for a scale of +9 (instead of the default of -9 because the fringe is toward the center of the frame). The result:
If you look hard, you can see a little residual fringe, which might be elminated by playing with the settings a bit, but which is definitely invisible in a print.
CAfree is copyright 2004 Thomas Fiddaman. Use and distribute it freely. Other rights reserved. Not for resale or commercial distribution without prior written permission.
CAfree includes absolutely no warranty express or implied - use at your own risk. Don't oversave your originals (duh).
Sorry, there's only a Windows version: CAfree 0.1
I'm continuing to work on this out of my own curiousity. Rather than emailing me, please contribute to the Sony forum discussion at DPreview - otherwise I'll get too distracted from real work. I'm particularly interested in pathological images that resist fixing but are somehow "worth saving". I find that most images with a lot of fringing are also bad in other ways; why waste time on those?