Purple Fringing questions.

Started Sep 28, 2006 | Discussions thread
Doug Pardee
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Re: Purple Fringing questions.
In reply to BobT, Sep 28, 2006

There are many different causes for "purple fringing". The only one caused by the lens is Chromatic Aberration (CA).

CA is a fact of life. I'm sure that you've seen the use of triangular prism to separate light into a rainbow of different colors. When you bend light, as through a prism or a lens, the colors separate. Lens manufacturers work very hard to keep the separation as small as possible.

You'll mainly notice CA on wide-angle lenses. The wider the angle, the more the light has to be bent as it comes into the lens. It's also harder to control CA on zoom lenses because the amount of light-bending varies from one focal length to another.

CA is more apparent in digital photography than in film because few people ever looked as closely at their film images as we do in digital. Blow a digital picture up to 100%, 200%, you'll see things that required inspecting an 8x10 with a loupe to spot when shooting film. The thing is, if you make a print the CA won't be any more visible than it was with film.

No, filters won't help if the problem is CA.

Yes, there are many techniques for eliminating many kinds of purple fringing in post-processing. For CA, you can use PTLens, DxO Optics Pro, Picture Window Pro, ACR's lens adjustments in PS/CS2, and I think that PhotoShop Elements 5 now has CA adjustments built-in. There are bound to be many more programs that I don't know about.

There are also techniques and plugins that can be used to render other kinds of purple fringing almost invisible. A Web search should help with that.

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