Purple flare: Causes and remedies

Started Mar 1, 2013 | Discussions thread
kenw
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Re: Hmmm... CC experiment implies non-linear process...
In reply to Cani, Mar 2, 2013

Cani wrote:

It is not clear to me why you think the result implies non-linearity. Could you elaborate on this?

It is a little obtuse, and Anders has already explained quite well what the most likely non-linear process involved is (color channel mixing).  But to answer your question the clue is that for any linear process you can not change the frequency (i.e. wavelength, color) of an input.  You can only change the magnitude of a frequency already present.  What Anders cleverly did was apply two offsetting linear filtering processes - one at the input with his CC filters and one at the output with the WB change.  These filtering processes are linear - they only change the magnitude of the wavelengths (frequencies) of light in the optical path - they can't make say red light change into blue light, they can only attenuate (the CC filter) or amplify (the WB change) different wavelengths.  For a linear process (one in which superposition holds) doing what Anders did should result in no color changes anywhere in the scene.  As an example if you take a photo with a WB altering filter in front of the lens (say a daylight to tungsten filter) and they apply the opposite WB correction in post you expect the exact same scene rendition as if you had shot without the the filter and WB correction.  If that didn't occur you know the process is not linear.  But as Anders has demonstrated, and this is where my reference was a bit obtuse, non-linear does not necessarily mean clipping or a non-linear tone curve.  At the heart of it, and the basis of my observation, was the fact that measured wavelengths of light changed relative to each other in a non-linear way.

Do you mean that for both versions of the second picture the results seem to violate superposition, but cannot and thus it's an artefact from an origin that remains to be determined? Or do you believe superposition can be violated?

Well, superposition certainly can be violated by any measurement system - something just needs to clip or be non-linear in some other way.  Or it can mix wavelengths of light as Anders has illustrated so nicely.  My point was that since we were seeing superposition violated then the root cause had to be at the sensor (just past the CFA it turns out) and not in the optics or AA filter.  Although of course in order for this sensor issue to appear you need off axis light and that light appears because of the lens...

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Ken W
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