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Mark
Wed May 9 2007 7:24 pm
 |  Chromatic Aberration - what is it?
One of our members (Teresa) recently submitted a photo having symptoms of Chromatic Aberration and has asked how to correct it.
CA (shorthand for Chromatic Aberration) is a lens anomaly that causes certain wavelengths of light to converge at different points on the film plane or digital sensor. The result is a color "shadow", usually red or blue, along edges of high contrast in the image. It's most often seen near the edges of a photo, but in severe cases it can affect the quality of the whole image.
Nearly all lenses suffer from CA to some degree, although careful optical design (such as the use of aspherical elements) can minimize the effect. As you'd expect, expensive lenses typically exhibit very little CA, while cheaper "consumer grade" lenses can have severe CA.
Since Chromatic Aberration is a lens characteristic, it can only be corrected in post processing (although using a smaller aperture can also reduce CA in most lenses). Photoshop includes a lens correction tool that shifts red and blue pixels to minimize the effects of CA in a photograph.
This photo shows red shift along the tree branch typical of CA. The inset shows a quick correction done in Photoshop.
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Westietess
Fri May 11 2007 8:31 pm
| Re: Chromatic Aberration - what is it?
Thanks for the information, Mark. Now I wish I had Photoshop!!!
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Lynda
Tue, 07 Aug 2007 17:51:24 EDT
 | Re: Chromatic Aberration - what is it?
You mentioned it on my moon shot too and I'm pleased you explained it as I couldn't see it. Now you've illustrated with Teresa's one what you mean I see it on mine bottom left....that's interesting as we have the same camera too.
Love Lynda XX.
 "A July Moon" by Lynda
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Mark
Tue, 07 Aug 2007 23:14:23 EDT
| Re: Chromatic Aberration - what is it?
Thanks for your comments, Lynda. Once you know what it is, you'll start to see CA everywhere you look, LOL!
Chromatic Aberration is quite common, even with some expensive lenses, particularly near the edges of photos. Luckily, you can often minimize CA by stopping down the aperture or using post-processing techniques as mentioned above.
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Lynda
Sat, 18 Aug 2007 14:40:00 EDT
| Re: Chromatic Aberration - what is it?
Now on my flat screen at work it showed up LOADS but not on my ancient screen at home !!
Love Lynda XX
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Mark
Sat, 18 Aug 2007 15:41:12 EDT
| Why a calibrated monitor helps
Yep, that's why I highly recommend monitor calibration ... you can't fix what you can't see :-)
On my (calibrated) monitor, the CA is very obvious. However, on my laptop display (not calibrated), I have to look very hard to see the halo.
There is another post entitled Monitor Calibration in the Public Forum which includes some helpful links.
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