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07-01-2008, 08:03 AM   #16
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In this example image you have some very noticeable CAs (green on the right hand side, and magenta on the left edge. So this explains the halo-effect you see. This seems to be on the pixel-level and will probably go unnoticed at standard print sizes. Its testimony (in my eyes, at least), that even the cheap kit lens does perform quite well in most condisitons. Your example is really a hard challenge for such a cheap lens and you will find CA or purple fringing with many lenses at this magnification level and with such a high contrast scene. I scrutinize my slides from time to time, and find, that I can also make out CAs, but they tend to get unnoticed, even when those slides are projected onto the wall. So, don't be shocked by just reachng the borders of your lens' performace...

Ben

07-01-2008, 09:54 AM   #17
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It is definately CA.

There are two forms of CA: Axial and lateral. PF is usually caused by axial CA, and what you are observing is caused by lateral CA. I have definately seen signs of lateral CA with the kit lens before.

The good news is that while purple fringing is impossible to fully correct in postprocessing (you can desaturate it, but that's not really "fixing" it), lateral CA can often be 100% fixed by scaling the red and blue layers independently of each other and the green layer. Many RAW converters allow this, some do it automatically based on EXIF and a lens properties database.

Edit: PF can occur at any part of the image, but lateral CA is usually far more pronounced at the edges of an image than at the center.
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