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02-20-2013, 01:59 AM   #1
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CA - couple of Qs

My background professionally is in physics, but I'm no optics expert.

But there seem to be two common misconceptions about chromatic aberration:

1) that CA can be reduced with a hood or by avoiding flare in general, and

2) that CA is easily corrected in software

In my view, #1 is false; flare is caused by light being scattered away from its intended path through the optics (which mostly ought to have ended up absorbed by black surfaces inside the lens or camera body) and has nothing to do with chromatic aberration, which is caused by the fact that each wavelength is refracted along a different path through the optics.

and #2 is false, because each colour channel has a certain spectral sensitivity- it is sensitive to a range of wavelengths. You can scale the images from the three channels to make them line up, but the different wavelengths that contribute to the intensity within each channel are going to blur the image in that channel at the edges in a way you can never undo. Roughly speaking, the spectral width of any channel is about half the spectral separation of red and blue - so for red/blue CA you can probably only ever remove about half of the blurring caused by red-blue CA. (Although potentially you could help undo the blurring in each channel using an unsharp mask with a radius that depends on distance from the image sensor. What software does this?)

I could be wrong?

02-20-2013, 02:21 AM   #2
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A lens hood improves overall IQ, which makes the CA less apparent, even if fringing and chromatic aberrations aren't removed. But the medium is also important, for example a lot of people say that fringing is more apparent on digital cameras than on film. So its hard to say CA is purely a lens property.
And regarding 2.. Sure? I mean, "easy" and "corrected" are both subjective, but certainly it can be affected and again, made less noticeable. With better and better algorithms, this can now be done quite well and will only improve in the future. For example, now there are even algorithms for fixing axial aberration, which didn't exist a couple years ago. Remember that photography is a visual art, its not about physics and absolute values.
Oh yes, but I also agree that CA can really ruin photos and I try to only use primes with minimal CA.
02-20-2013, 04:00 AM   #3
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QuoteOriginally posted by Na Horuk Quote
I also agree that CA can really ruin photos and I try to only use primes with minimal CA.
That is another fallacy - that primes automatically have less CA than zoom lenses do - in the 1970's there wasn't anything wrong with that assumption, back then zoom lenses were terrible. However these days there are certain cases where zoom lenses are in fact markedly superior to primes lenses at controlling optical aberrations.

From my comparison of the Sigma 8-16mm f/4.5.-5.6 ASPH Vs the DA15mm f/4 ED ASPH:


100% unsharpened crops from the Pentax K5IIs

Sure it is possible to remove chromatic aberrations, but the sharpness of the lens isn't improved by the correction - because the sensor has already captured all that the lens can resolve.


100% unsharpened crops from the Pentax K5IIs - the crop from the Pentax DA15mm has been sharpened and lens corrections have been applied.The crop from the Sigma 8-16mm lens has been untouched.

In fact I think that resolution from lenses is inhibited to an extent by software lens corrections - because the channels have to be resized and interpolated to correct the optical aberrations in the first place - and that interpolation can affect image quality.

The sigma 8-16mm lens has 15 elements in 11 groups, the Pentax DA15mm f/4 ED ASPH has 8 Elements in 6 groups. The sigma lens has almost double the amount of elements in it, when technically the DA15 should be better corrected due to being a single focal length lens and the use of ED glass and aspheric lenses.

Last edited by Digitalis; 02-20-2013 at 04:12 AM.
02-20-2013, 12:04 PM   #4
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QuoteOriginally posted by Digitalis Quote
certain cases
Exception proves the rule. Obviously, if you have a poor prime and a great zoom the zoom has a chance, but clearly thats not what I meant. You can always find outlier lenses or stage absurd comparisons to prove a point.

02-20-2013, 12:51 PM   #5
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QuoteOriginally posted by russell2pi Quote
But there seem to be two common misconceptions about chromatic aberration:

1) that CA can be reduced with a hood or by avoiding flare in general
I thought I heard all common misconceptions, but this one is new to me.

QuoteOriginally posted by russell2pi Quote
2) that CA is easily corrected in software
It depends on the CA - some CA can get easily corrected in software. I removed lateral CA very effectively from a DA15 shot using the Pentax software and its default CA correction for the lens.

Corrections can be effective enough that you do not need to worry about CA. And correcting CA is very expensive - it requires using a more complex optical formula with more glass elements or special glass, with the result that lenses become larger, heavier, more expensive, and then people wonder why they are priced that way. When Leica announced the APO Summicron 50/2 lens priced at $7,195, some people wondered why it is so expensive compared to the "affordable" $2,295 Summicron 50/2 - the answer was in those three letters: APO.
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