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Old Sep 16, 2003, 1:36 AM   #1
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Default Aboritions and halos

Hi All: I remember seeing a discussion sometime back about aboritions or halos in digital shots. I can't find the posts again so here we go.... What was the cause of these purpleish pink outlines around items and what is the solution? Thanks in advance,
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Old Sep 16, 2003, 1:45 AM   #2
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Default Re: Aboritions and halos

Quote:
Originally Posted by Mike Haywood
aboritions....
Obviously you need to visit an aborition clinic.
I didn't see the thread on "aberration", but if you search on that word you might well find it. Good luck!
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Old Sep 16, 2003, 6:11 AM   #3
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FYI

http://www.stevesforums.com/phpBB2/v...177&highlight=
... A possible fix is described toward the bottom of the last link...
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Old Sep 16, 2003, 7:54 PM   #4
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Chromatic abberation is condition causing a purple fringe around the outer edges of some subjects in a photo, usually when using a high powered zoom. The situation most commonly occurs with dark objects against light backgrounds (e.g. tree branches against a bright sky).

The condition can be minimized when taking the picture by avoiding sharp contrast at long zoom range and the results can be mitigated with a photo editor, but not eliminated.

My Canon Pro90 has this problem, but it can be minimized.
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Old Sep 17, 2003, 4:04 AM   #5
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Default Try the fix

Just by coincidence I was over on dpreview a little while ago and read a thread that pointed to the same fix.
http://www.dpreview.com/learn/Glossa...rations_01.htm
That is the link to the fix, not the thread I was reading. I tried it with one pic and I thought it worked great. It does not get rid of the purple fringing but rather converts it to grey so it is much less visable. I'm not sure if it will help with all photos but the one I tried it with was much improved.
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Old Sep 17, 2003, 9:20 AM   #6
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Though any "weirdness" of color in one's image might be called an "aberration" true "chromatic aberration" has its origins in the lens system of the camera.

When light passes through a transparent medium, it's sometimes broken into the frequency spectrum we see as different colors. Think of light through a prism. The purpose of a lens is to focus this light on the plane of the film or sensor, so various coatings and multiple elements are used in lens technology to achieve a goal of getting these various frequencies to arrive at the same place at the same time.

When this fails to happen, we get a condition of color fringing known as "chromatic aberration." Technically, this is seen as red/green (most of the time) fringing in the periphery of the image. Another condition mentioned in on the the posts where a purple/blue fringe occurs at junctures of high contrast (green leaves meeting blue sky, etc.) is usually caused by cross sensor current leakage (between adjacent photosites on the CCD) and is properly called "blooming."

Most zoom lenses on consumer/prosumer cameras will produce some degree of chromatic aberration, especially likely if the zoom is greater than 3x. Professional lenses use low dispersion glass and specialized coatings to control this, but the expense of doing this takes it out of the realm of probability for consumer equipment.

True chromatic aberration can be ameliorated by various software algorithms (I use and recommend Picture Window Pro 3.1) which can "correct" all but tiny amounts which are indistinguishable in print. Blooming is best avoided by being very careful to not overexpose, especially in areas of high contrast. In the cases when it's not possible to avoid it, one can selectively desaturate the areas affected and produce quite significant improvements.

Lin
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Old Sep 17, 2003, 2:15 PM   #7
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Would this be the feature you are refering to in picture window pro?
"Moire Reduction: The latest addition to PW artifact correction functions. Removes annoying color patterns which arise when image features interfere with the pixel grid in the CCD sensor "
Ver 3.5 is what they have right now so I am not sure if it has the same functions. Can this software be gotten through resellers or just direct?
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Old Sep 17, 2003, 2:59 PM   #8
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Hi Richard,

No, moire is a different situation. Moire is generally found when there is a regularly occurring pattern such as found in a tweed or herringbone suit or screen wire, etc., which causes strange color artifacts when photographed from specific distances. Watch TV during one of the sports presentations and when the camera moves in and out you can frequently see strange splashes of colors on the suits worn by the commentators. This is moire. Digital cameras with weak antialiasing filters like the Canon EOS-1D tend to create moire especially when photographing people or fabrics. Having a moire filter in the new version of Picture Windows Pro is a nice addition.

There is a specific portion of Picture Windows Pro dedicated to removal of chromatic aberration. It's done visually with two sliders which adjust red/green and blue aberration. It won't fix "blooming," but does a great job with true chromatic aberrations. Picture Windows Pro is a great program written by one of the last great individual software developers (the man who wrote Lotus 1-2-3). It's well worth the money, but you do have to buy it direct from the developer.

Best regards,

Lin
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Old Sep 17, 2003, 10:01 PM   #9
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Hi-
Many thanks to those who responded. I have tried your suggestions and gone to suggested links and made great improvements. It was my son's wedding and while I was not the photgrapher I did want the ones I took to look decent for viewing. The white things such as Tuxedo jackets and white flowers againts a dark backgound were the offenders. Your suggestions helped neutralize the effect to an acceptable level.
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Old Sep 18, 2003, 2:13 AM   #10
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Thanks for the information Lin. It is a recentlly purchased teleconverter that is the offender. Picture window pro 3.5 has a free 30 day trial so I think I will wait till I have a few more offensive pics and give it a try.
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