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Old Jul 4, 2005, 1:54 AM   #1
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Gents,

what do you think of the sample below? As I understand it, the blue line along the flower is CA. Am I right/wrong...? Can I do anything about it? Is it "normal" CA?

I usedEF-S 17-85mm on 350D. I also have to work on my manual focus...

thanks for your comments!

JM
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Old Jul 4, 2005, 1:55 AM   #2
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here is the full shot... I guess the yellow flowers in the background should not be there!
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Old Jul 5, 2005, 10:34 PM   #3
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Yes, in my opinion that is CA.

CA is created by cheap lenses that don't have the (expensive) multi-coatings that can prevent it. At least, I think the multi-coatings prevent it. But its part of what you get with cheaper lenses. In my experience, brightly lit things also show it more than darker scenes.

There are various software packages that claims to reduce it... I don't know if they really do or not, I haven't used any of them.

Eric
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Old Jul 6, 2005, 1:28 AM   #4
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hum... to be honest I did not consider EF-S 17-85mm f4-5.6 IS USM cheap...

but according to certain threads, only L is worth it...

thanks for the answer though
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Old Jul 6, 2005, 5:43 AM   #5
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Chromatic Aberration is caused because different wavelengths of light (different colours) diffract by a different amount when they pass through lenses. It can be corrected by using more than one lens element - one to bend the light in to focus it and one to bend it out again. If the two elements have different diffractive properties you can still focus the light while removing the CA. You can also use materials such as low dispersion glasses that cause less of it in the first place, special lens coatings or a combination of these.
Purple fringing is also caused when light spills between pixels. You have one black and one light pixel and the one in between is a bit of both. The camera doesn't know that it should be either black or white and so it ends up somewhere in between (most often purple). This can be made worse if you are boosting noise somehow (using high iso for instance or making lower quality jpegs) or by using the lens wide open where it's not at it's sharpest. Exactly when, where and why this happens doesn't seem to be a precise science.
You might do better with a different focal length or by stopping the lens down a bit or with brighter light.
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Old Jul 6, 2005, 5:52 AM   #6
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jacks is entirely correct (ditto with the pixels part):
http://www.dpreview.com/learn/?/Glos...rration_01.htm




polarwasp wrote:


Quote:
but according to certain threads, only L is worth it...
Are you kidding???
'L' have CA too! :roll:
http://www.stevesforums.com/forums/v...=65&page=2
http://www.stevesforums.com/forums/v...mp;forum_id=37

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Old Jul 6, 2005, 7:32 AM   #7
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well, I'm kidding actually ... :-):G:blah:

I'm perfectly happy with the 17-85mm, thank you! I just wanted to learn more about my kit so that I can make the best of it! And that is much learning in this thread so far!

I got the SIGMA 70-300 APO recommended in this forum and it fits perfectly my needs as well... And my next lensis a SIGMA macro... still hesitating between 105mm and 150mm but I am learning with the macro capability of the 70-300... Sorry I am a bit sidetracked...

Thank you guys, I appreciate your comments.
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