Purple flare: Causes and remedies

Started Mar 1, 2013 | Discussions thread
Anders W
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Re: Excellent!
In reply to kenw, Mar 2, 2013

kenw wrote:

Very well thought out and demonstrated Anders!

Thanks. I am glad the theory passed at least first scrutiny.

I now completely understand why you tried your CC filter experiment and why it worked. Nicely done. Yes, there is non-linearity in there - not the way probably most people think about non-linearity (clipping) but exactly the way some one who does signal processing (me) thinks about non-linearity (frequency mixing).

I can't think of a better explanation than the one you offered. I might also add for others clarification that the same reason that Anders describes color imbalance resulting in purple/magenta coloration in this case is also the reason shadow noise is magenta in color in high ISO shots. And as a very far aside the same reason a camera converted to IR with a long wavelength filter (830nm) results in magenta images.

Yes. One might add here, again for clarification to others, that while the color-channel imbalance is an essential ingredient in the purple flare as well as the purple shadow noise we might see in high-ISO shots (and low-ISO shots if the shadows are strongly pushed in PP), the other essential ingredient differs: what I call color-channel pollution in the former case and clipping of the read-noise distribution in the latter.

I can see another excellent application of your very clever CC testing technique - purple fringing. In almost all cases I've seen it is fairly clear most purple fringing is just longitudinal CA. There are, however, a few times I've seen things that might not be that and of course there are many pet theories as to what causes it beyond longitudinal CA (some clearly wrong, such as blooming, and others more believable and closely related to what you demonstrate here). Your CC test would of course also sort out those cases in which purple fringe is from the expected CA and which cases something more interesting at the sensor/microlens/CFA is occurring.

Yes, you are right about that. The CC technique might be useful in settling some old disputes about lens or sensor as the cause of fringing of this or that kind. If it's the lens, altering the channel balance should have no impact. If it's the sensor, it should.

Oh, one last thought and aside. I haven't thought about it long enough, so I could be wrong, but given we need off axis light to cause the purple and that light is coming from a reflection somewhere in the lens it seems to me that the reflection causing it could only come from a reflection that occurs between the aperture stop and the sensor (i.e. reflections in front of the aperture stop can't turn purple as it isn't possible for them to be off axis).

Not sure I follow you entirely here. You may well be right but could you elaborate a bit on your idea that a reflection prior to the aperture stop could not pass the stop and subsequent lens elements to finally arrive at the sensor at an angle of incidence greater than that possible for anything the lens actually images?

Finally, do you have any further ideas about purple flare in the diagonal versus horizontal/vertical direction? An idea that has occurred to me is that the horizontal and vertical but not diagonal pattern shown by my second series of test shots might be due to the pollution being possible only between pixels that are in the same row or column but blocked diagonally. On the other hand, we have plenty of examples of purple blobs that do not take on this pattern but I cannot at present rule out the possibility that they could look the way they look even if diagonal pollution wouldn't be possible. Optical pathways are obviously pretty tricky to sort out in cases like these.

At any rate, I will try to see what happens if I try to form and then get rid of (by means of the polarizer) purple streaks like those in the first series of test shots but with the camera tilted so that the streaks become diagonal rather than vertical.

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