Purple flare: Causes and remedies

Started Mar 1, 2013 | Discussions
Timur Born
Timur Born Veteran Member • Posts: 4,763
Re: Purple flare: Causes and remedies

Didn't you write that you also kept ISO unchanged? This is why I asked for tone-curves (to apply digital gain for lower analog exposure/gain). Something along the lines of clipping the red/blue channels less?

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OP Anders W Forum Pro • Posts: 21,468
Re: Purple flare: Causes and remedies

Timur Born wrote:

Didn't you write that you also kept ISO unchanged? This is why I asked for tone-curves (to apply digital gain for lower analog exposure/gain). Something along the lines of clipping the red/blue channels less?

Yes, I kept ISO unchanged along with f-stop and shutter speed. And as I have already revealed (see my post just prior to this one), I used a light-reducing filter not only for image two but also image three. So I adjusted the exposure slider in PP (LR 4) to keep all three shots at the same brightness. So more gain, but no change of tone curve.

When you get very close to zero signal, you might get color shifts due to the read noise (but not the signal) being clipped by the black-level offset. But no, there is no problem with that here. We are not that far down.

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secretworld Senior Member • Posts: 1,702
Re: Purple flare: Causes and remedies

OMG this is killing me. I want to know and I don't even own a Oly (I do own the 7-14). Is the third a variable neutral density filter?

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OP Anders W Forum Pro • Posts: 21,468
Re: Purple flare: Causes and remedies

secretworld wrote:

OMG this is killing me. I want to know and I don't even own a Oly (I do own the 7-14). Is the third a variable neutral density filter?

No. Someone already suggested that. I have already revealed that both image two and three use filters. In the case of image two, it is a polarizer as already guessed by several people (first by Macx), more specificially a linear one.

But what filter was used for image three? Noone has come close to the right answer to that question yet.

And that filter and its effect is actually of greater interest (I think) for the purpose of understanding what causes the purpleness in the first place. The effect of the polarizer primarily tells us that the flare as such is due to reflections somewhere, and we kind of knew that from the outset.

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H J B New Member • Posts: 2
Re: Purple flare: Causes and remedies

Anders W wrote:

Bizzarrini wrote:

On no. 3 I notice some increased noise or maybe just JPG artifacts and the file is Bigger in size.

Could be a bit more noise, yes. No special reason for more jpeg artifacts nor for a bigger file size.

Did you use an ND filter?

No, not an ND filter. But to help you all a little bit, I did use a filter not only for image two (polarizer) but also for image three, and the filter reduced the amount of light passed on to the sensor (as most filters do), which in turn explains why there is more noise (since I kept f-stop and shutter speed unchanged).

Was it a yellow filter?

kenw
kenw Veteran Member • Posts: 5,354
Not a Haze 2A or 1A?

Clever use of the polarizer on #2.

So for number three this isn't something like a Haze 1A or 2A getting rid of the shortest wavelengths?

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OP Anders W Forum Pro • Posts: 21,468
Re: Not a Haze 2A or 1A?

kenw wrote:

Clever use of the polarizer on #2.

Thanks!

So for number three this isn't something like a Haze 1A or 2A getting rid of the shortest wavelengths?

Well it would get rid of the shortest wavelengths allright. But that doesn't help. The problem is not that the built-in (on-sensor) UV filter is too weak.

Rather I went in exactly the opposite direction. "Ont skall med ont f├Ârdrivas" (fight evil with evil) as we say in Sweden. Here's what image three looks like before proper WB, i.e., when using the ordinary tungsten WB with which it was shot. Now why does such a filter (there are actually three of them on top of each other) help?

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OP Anders W Forum Pro • Posts: 21,468
Re: Purple flare: Causes and remedies

H J B wrote:

Anders W wrote:

Bizzarrini wrote:

On no. 3 I notice some increased noise or maybe just JPG artifacts and the file is Bigger in size.

Could be a bit more noise, yes. No special reason for more jpeg artifacts nor for a bigger file size.

Did you use an ND filter?

No, not an ND filter. But to help you all a little bit, I did use a filter not only for image two (polarizer) but also for image three, and the filter reduced the amount of light passed on to the sensor (as most filters do), which in turn explains why there is more noise (since I kept f-stop and shutter speed unchanged).

Was it a yellow filter?

No, it was this.

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secretworld Senior Member • Posts: 1,702
Re: Purple flare: Causes and remedies

Oh please tell us, I have a cold and can't think of anything logical.

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OP Anders W Forum Pro • Posts: 21,468
Re: Purple flare: Causes and remedies

secretworld wrote:

Oh please tell us, I have a cold and can't think of anything logical.

But I just told you what kind of filter I used. Didn't you see the link in the post to which you responded?

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secretworld Senior Member • Posts: 1,702
Re: Purple flare: Causes and remedies
1

Yes I saw it, but no; why you think it works...

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OP Anders W Forum Pro • Posts: 21,468
Re: Purple flare: Causes and remedies

secretworld wrote:

Yes I saw it, but no; why you think it works...

I'll reveal that in due time.

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Cani
Cani Regular Member • Posts: 386
Re: Not a Haze 2A or 1A?

Ok. You went up "the opposite direction" and you end up with a picture all purple... All purple means a mix of blue and red and no green, right? Green is in between blue and red.

3 filters... I am not familiar with filters. UV filter is a low band pass-through filter I believe. How do red, blue and green filters work? You need to withdraw green...

Actually you would need a magenta filter to absorb green. How do you build a magenta filter with 3 filters...

Still searching...

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OP Anders W Forum Pro • Posts: 21,468
Re: Not a Haze 2A or 1A?
1

Cani wrote:

Ok. You went up "the opposite direction" and you end up with a picture all purple... All purple means a mix of blue and red and no green, right? Green is in between blue and red.

Right.

3 filters... I am not familiar with filters. UV filter is a low band pass-through filter I believe. How do red, blue and green filters work? You need to withdraw green...

Yes, I am trying to limit green and strengthen red and blue, especially blue which in this case is by far the weakest channel (since the light is artificial, halogen bulbs, at roughly 3000 K).

Actually you would need a magenta filter to absorb green. How do you build a magenta filter with 3 filters...

I didn't have to build a magenta filter. Two of the filters were magenta filters: A Tiffen CC30M and a Formatt CC70M (the strongest magenta filter I have been able to find) for a combined effect of CC100M. This filter combination favors red and blue at the expense of green. On top of that I used a Hoya HMC 80A, i.e., the blue filter used in the film days (I think I bought the filter some 35 years ago) when you wanted to shoot color film balanced for daylight with 3200 K photo bulbs. This filter favors blue at the expense of green (to some extent) and red (to a greater extent).

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Timur Born
Timur Born Veteran Member • Posts: 4,763
Re: Purple flare: Causes and remedies

Anders W wrote:

Timur Born wrote:

Shot as Jpg or Raw? Tone curves involved?

RAW. Processed in LR. No tone curves involved.

My fault. When was write "tone-curves" I mean anything from linear to non-linear (LR's exposure slider isn't linear anyway, is it?), so I wasn't far off with my suspicion that the increased noise meant digital gain.

What happens when something purple is in the scene and would that filter and necessary gain babe preferable to using LR's defringing instead?

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OP Anders W Forum Pro • Posts: 21,468
Re: Purple flare: Causes and remedies

Timur Born wrote:

Anders W wrote:

Timur Born wrote:

Shot as Jpg or Raw? Tone curves involved?

RAW. Processed in LR. No tone curves involved.

My fault. When was write "tone-curves" I mean anything from linear to non-linear (LR's exposure slider isn't linear anyway, is it?), so I wasn't far off with my suspicion that the increased noise meant digital gain.

What happens when something purple is in the scene and would that filter and necessary gain babe preferable to using LR's defringing instead?

If there would be something genuinely purple in the scene (i.e., a subject with purple color rather than flare in the optical path) it would be rendered purple even with the purple filter after proper WB was set. So the filter reduces the false purple that isn't really part of the scene but would leave genunine purple intact.

As to LR's defringe tool, it is not an ideal tool for removal of the false purple on several grounds: because LR doesn't see the flare as a fringe and therefore has limited effect and/or because its impact is to desaturate which replaces one artificial/undesirable hue by another.

Local change of WB (using LR's brush tool) is usually a better bet than the defringe tool in my experience. But the point here is that the filter counteracts the cause of the purple disease in the first place rather than attacks its symptoms afterwards. That said, I regrettably don't think that I have discovered some miraculous cure-all, nice as that would be, since heavy filtering has costs of its own (not least the loss of several stops worth of light). I am in fact more interested in the effect of the filter because of what it tells me about the cause of the purpleness than as a practical tool in the field.

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idiotekniQues
idiotekniQues Senior Member • Posts: 1,255
nice experiment, but not practical. no thank you
1

best solution? if you get more purple flare than you want?

buy a panny body if you want the 7-14 more than the Oly body.

buy the Oly 9-18 if you want to keep the Oly body more than the 7-14.

that's my solution.

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OP Anders W Forum Pro • Posts: 21,468
Re: Purple flare: Causes and remedies
1

Here is another set of examples using the linear polarizer. While producing them, I also took the opportunity to verify that, as expected, a circular polarizer has no impact at all.

As you can see, I have in this case directed the camera directly at a prominent light source in the form of a spotlight in my living room. Again, the first shot shows what things look like without a filter. As you can see, there are rather prominent streaks of purple horizontally and vertically but not diagonally. If I recall correctly, it was Timur Born who first noted this peculiarity, and it is conceivable that this particular pattern provides a clue with regard to the ultimate cause of the purple disease.

Image two and three are both shot with the polarizer and as you can see, it is possible to eliminate the purple vertically or horizontally but not both simultaneously since the vertical streak is polarized in a different way than the horizontal. This is one reason why I told MAubrey not to be too optimistic about using the polarizer as a panacea. In the first series of shots, the polarizer pretty much eliminated all of the purpleness but, as shown here, it may not do so under less benign circumstances.

´╗┐

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OP Anders W Forum Pro • Posts: 21,468
Re: nice experiment, but not practical. no thank you
1

idiotekniQues wrote:

best solution? if you get more purple flare than you want?

buy a panny body if you want the 7-14 more than the Oly body.

buy the Oly 9-18 if you want to keep the Oly body more than the 7-14.

that's my solution.

The problem with that solution is that any lens, including the 9-18, can give rise to purple flare on the E-M5 (and perhaps other Oly bodies). See here with respect to the 9-18, although the purple illustrations have regrettably been removed:

http://forums.dpreview.com/forums/post/50710489

In general though, I think the present thread is of greater interest for those who want to understand what's going on irrespective of how much of a practical solution it provides.

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kenw
kenw Veteran Member • Posts: 5,354
Hmmm... CC experiment implies non-linear process...
1

Thanks for sharing this Anders.

The polarizer does strongly imply reflection.  Keep in mind though the AA filter depends on polarization as well, and your most recent experiment showing the horizontal and vertical flares implies something not rotationally symmetric and aligned with the AA filter and pixel array is at least partially at play.  Would be curious to see a polarizer test with a diagonal purple flare to narrow down the likely causes...

The bigger question to me is the CC filter result.  It implies a non-linear (i.e. violating superposition) process is occurring.  Such things are exceedingly rare in optics as superposition is a basic tenant of E&M theory.  Immediately attention is drawn to the sensor itself or the post processing (recovering WB and exposure compensation) as the only likely sources of a non-linearity.  Do you have a theory for what is going on with the CC filter experiment?

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