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-   -   CA and PF correction -- the best way I've discovered so far (

snostorm Jul 12, 2007 7:56 PM

Hi All,

I was playing around today, trying to find a better solution to Purple Fringing and Chromatic Aberration, and discovered something that will work with both Photoshop and Paint Shop Pro (vers 9 and up) -- might work with other programs, but I don't use any others.

It's not a one click solution -- the ones I've used haven't really satisfied. . . but it's results are the best, IMO.

Here's a 100% crop of a recent pic of a Robin's head against a sky background and an out of focus branch in the background. It shows PF on the robin's head, and green CA on the branch -- there's also some additional blue CA under the robin's beak hidden under the more apparent PF.

In the second sample, the same crop, but I desaturated the PF by going to the blue channel, desturating by about 70% and adjusting the hue of the channel towards the magenta end until the PF pretty much disappeared.

Not bad, but it turned the underlying image elements gray, mostly notable on the beak -- but now the blue CA shows on the underside of the beak as well as the feathers below it -- and the branch is still fringed with green. If I desat the blue, it'll change the color of the sky also. Most of the PF and CA tools do essentially the same thing, just desaturate the colors that you want to eliminate and most will effect the other colors to some extent.

Here's the same crop with the new fix.

I did this by using the clone brush only -- but changed the blend mode to color -- this clones the hue and saturation only, leaving the brightess and details of the underlying image elements alone. I used a relatively large brush, hardness at @ 20, and opacity at @ 60. You don't have to be careful about the edges. It'll change the color of the background, but not by too much, and this can be corrected easily. It doesn't matter where you choose your source, as long as the hue is in the ballpark.

An example is the head -- you can choose the dark part as the source -- it only changes hue and saturation, not the lightness on the cloned area. Go over the area to get rid of the purple. This can be used to get rid of all the different unwanted colors, so the underlying blue CA is taken care of at the same time as it gets rid of the PF on the beak. The same brush can be used on the branch, just take the source from the sky

If you've been a little sloppy, cut the opacity down to @ 20, and choose your source from the background color, again you don't have to be too careful with the edges. The lower opacity should correct any changes made by the first operation without being too obtrusive to the subject.

My wordy explanation might look complicated, but it's really easy. Not as easy as the one click solutions, but it leaves all the other colors unchanged as well as the image details.

Give it a try, and tell me what ya think. . .


VTphotog Jul 12, 2007 8:56 PM

Nice job. I have used the PFree plugin, which works fairly well, but is difficult to master, and ends up taking me more time than I like to get it right.

The method I hit upon is to use Color Replacement , which works the same in both PSE 3 and Photoimpact. Not sure about PSP. Select the CA or PF (one at a time is best) and adjust sensitivity for just what you want to change, then use the hue and saturation sliders to match color to background. Lightness setting can further blend it in. This is the quickest method I have found to date.

Shooting RAW, and using Rawshooter, I find that it seems to eliminate most PF in the conversion stage, so I don't have to deal with it at all. Raw Therapee also has a CA adjustment, which seems it could be setup for each lens as needed.


rhermans Jul 13, 2007 12:26 AM

Scott, Thanks.....

It works as a charm - and is much easier than any I've tried before.


jachol Jul 13, 2007 3:23 AM

That sounds very logical Scott, and I'm sure it will work in PS. I've nothing I can try it on at the moment, but have made a note of the process to try later. Thanks for the heads up. ... Jack

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