Purple flare: Causes and remedies

Started Mar 1, 2013 | Discussions thread
kenw Veteran Member • Posts: 5,350

Very well thought out and demonstrated Anders!

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.

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.

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).

Again great work!

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Ken W
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