Purple flare: Causes and remedies

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

Anders W wrote:

kenw wrote:

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?

Well I should really add an important qualification to that now that I think about it.  I was considering the case more of something like the 7-14 or for the 45/1.8 it being stopped down significantly.  In those cases the exit pupil is rather small compared to the rear element of the lens.

All image forming light, from the perspective of the sensor, originates from the exit pupil - i.e. the image of the aperture stop.  I assume by design light originating from the exit pupil won't cause "purple effects", the lenses are semi-telecentric for this reason.  All reflections prior to the aperture stop of course can only be seen through the exit pupil and thus wouldn't cause "purple effects" either.  (Please also see an import caveat in the answer to your second question).

Reflections past the aperture stop, however, could appear at any angle from the sensor within the confines of the rear element.  That is to say the sensor would see the exit pupil and then well off axis from the exit pupil a reflection from elements on the sensor side of the aperture stop.

It is hard to say for sure, but I'm noticing in the "purple orbs" examples I think most of those blobs show aperture blades which means they are reflections past the aperture stop and jive with my half-baked theory...

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.

So I wonder if in your two examples (light just outside FoV and light dead center) we are in both cases seeing purple because of color channel pollution and color channel imbalance but that the optical cause of the pollution is different in the two cases.  That's sort of why I was curious about a purple diagonal flare from an outside the FoV source.  If you can generate (and correct) that it would seem to imply the horizontal/vertical pattern you see in the centered light example is from a different optical effect or reflection (and because of the vertical/horizontal effect something more related to the sensor or AA filter as the source of the polluting light).

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Ken W
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