Purple flare: Causes and remedies

Started Mar 1, 2013 | Discussions
Anders W
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Re: Purple flare: Causes and remedies
In reply to MAubrey, Mar 1, 2013

MAubrey wrote:

Anders W wrote:

Macx wrote:

Well, I suppose it isn't a lens hood.

Pol-filter perhaps?

Yes, in one case. Which? And for what reason can a polarizer remove lens flare and/or its purpleness?

Hmmm. So at least one answer is DIY filters for the 7-14mm...

Yes or the WonderPana from Fotodiox.

http://www.dpreview.com/news/2012/12/06/Fotodiox-launches-WonderPana-Filter-Systems-145-66

However, I wouldn't jump to the conclusion that a polarizer always helps as well as it did in this particular case.

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Anders W
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Re: Purple flare: Causes and remedies
In reply to richarddd, Mar 1, 2013

richarddd wrote:

Anders W wrote:

richarddd wrote:

I'd note that there are two issues, flare and purple. Purple is relatively easy to deal with in post processing (if you have the right tools). Flare is a much more difficult task in post.

Yes, but the purpleness is not always that easy to get rid off, depending on how much of it and where it is. I can't remember seeing any complaints about flare with the 7-14 before that flare started to become purple. The reason, of course, is that before that it was hardly visible since it merely implied a slight loss of contrast, although it was certainly present.

Perhaps because people are generally used to flare, especially with an UWA lens, while purple flare is much more unusual.

The usual remedy for flare is to avoid bright lights, especially point sources.

Yes, but there are quite a few scenes where this is more or less impossible.

Yes, alas.

I look forward to seeing your technique.

I hope I won't make you too disappointed. But I should mention in advance that I think my main contribution (if any) will be toward a better understanding of the causes. Not that the solutions are bogus but they do have their downsides.

I notice you didn't include EXIF data. Does that provide a clue to your technique?

They might, yes, at least in one case and perhaps indirectly in the other. So I removed them in order to make the quiz a bit more challenging.

If it's in the EXIF data and it's not aperture, that leaves shutter speed and ISO, neither of which I'd have thought would have any effect on flare, purple or otherwise.

The EXIF contains a lot of stuff as you can see if you have the right EXIF reader. No, I mentioned that I kept shutter speed constant (1/5 s) and ISO stayed constant at 200.

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Anders W
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Re: Purple flare: Causes and remedies
In reply to Macx, Mar 1, 2013

Macx wrote:

Anders W wrote:

Macx wrote:

Well, I suppose it isn't a lens hood.

No.

Pol-filter perhaps?

Yes, in one case. Which? And for what reason can a polarizer remove lens flare and/or its purpleness?

2nd one? Well, the flare is a reflection and as such polarised light? *guessing*

It's image two of three, yes. And yes, you are on the right track. Could you develop your thought a bit and what kind of polarizer is required in this case?

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Sanpaku
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Re: Purple flare: Causes and remedies
In reply to Anders W, Mar 1, 2013

The EM-5 sensor, as exceptional as it may be, has a coating on its attached hot mirror that reflects purple light off axis:

If the rear element of an attached lens is small, highly convex, recessed into the lens body, or has a multicoating that effectively squelches reflections all the way to ends of the visible spectrum, purple flare isn't much of a problem.

Unfortunately, the 7-14 has a large, only slightly convex rear element with a coating that bounces off-axis reflections from the sensor back onto it.

This forum looked in the issue in detail months ago, and short of replacing the anti-reflective coatings on either the hot mirror or the 7-14 rear element, there doesn't seem much to be done except to recompose so the purple splotches are not problematic.

I'm sure Olympus is aware of the issue, though images I've seen of the the E-M2 and and E-PL5 indicate they're still using the same hot mirror.

Now I've read Ander's post, its a linear polarizer, right?  That should cancel out polarized reflections from the sensor.

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Anders W
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Re: Purple flare: Causes and remedies
In reply to Erick L, Mar 1, 2013

Erick L wrote:

Anders W wrote:

So what did I do? Two different solutions were tried, one for image two and another for image three.

So, what did you do?

Part of the answer has already been revealed (see prior posts by Macx). But I thought I'd keep you guessing a bit before revealing the whole story.

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Anders W
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Re: Purple flare: Causes and remedies
In reply to Sanpaku, Mar 1, 2013

Sanpaku wrote:

The EM-5 sensor, as exceptional as it may be, has a coating on its attached hot mirror that reflects purple light off axis:

If the rear element of an attached lens is small, highly convex, recessed into the lens body, or has a multicoating that effectively squelches reflections all the way to ends of the visible spectrum, purple flare isn't much of a problem.

Unfortunately, the 7-14 has a large, only slightly convex rear element with a coating that bounces off-axis reflections from the sensor back onto it.

This forum looked in the issue in detail months ago, and short of replacing the anti-reflective coatings on either the hot mirror or the 7-14 rear element, there doesn't seem much to be done except to recompose so the purple splotches are not problematic.

I'm sure Olympus is aware of the issue, though images I've seen of the the E-M2 and and E-PL5 indicate they're still using the same hot mirror.

Yes, I remember we discussed that possibility. But that (reflections off the sensor, back on the rear element, and then back again) is not what I think is going on. Note that didn't use the 7-14 in this case. Also note that if you inspect the E-M5 sensor with your eyes, the reflections you see are redish and greenish, not purple (no real sign of blue).

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MAubrey
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Re: Purple flare: Causes and remedies
In reply to Anders W, Mar 1, 2013

Anders W wrote:

MAubrey wrote:

Anders W wrote:

Macx wrote:

Well, I suppose it isn't a lens hood.

Pol-filter perhaps?

Yes, in one case. Which? And for what reason can a polarizer remove lens flare and/or its purpleness?

Hmmm. So at least one answer is DIY filters for the 7-14mm...

Yes or the WonderPana from Fotodiox.

http://www.dpreview.com/news/2012/12/06/Fotodiox-launches-WonderPana-Filter-Systems-145-66

However, I wouldn't jump to the conclusion that a polarizer always helps as well as it did in this particular case.

Don't worry. I always prefer walking to conclusions rather than jumping. In this case, I'm most definitely awaiting for the full unveiling.

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Sanpaku
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Re: Purple flare: Causes and remedies
In reply to Anders W, Mar 1, 2013

Yes, I remember we discussed that possibility. But that (reflections off the sensor, back on the rear element, and then back again) is not what I think is going on. Note that didn't use the 7-14 in this case. Also note that if you inspect the E-M5 sensor with your eyes, the reflections you see are redish and greenish, not purple (no real sign of blue).

It occurred to me that if one used a linear polarizer on the lens, it would automatically reduce reflections (from certain angles) off the sensor.  Is that what is in your images?

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Macx
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Re: Purple flare: Causes and remedies
In reply to Anders W, Mar 1, 2013

Anders W wrote:

Macx wrote:

Anders W wrote:

Macx wrote:

Well, I suppose it isn't a lens hood.

No.

Pol-filter perhaps?

Yes, in one case. Which? And for what reason can a polarizer remove lens flare and/or its purpleness?

2nd one? Well, the flare is a reflection and as such polarised light? *guessing*

It's image two of three, yes. And yes, you are on the right track. Could you develop your thought a bit and what kind of polarizer is required in this case?

Well, since it's reflections from light that's already passed through the filter, a simple filter like the ones in polaroid sunglasses could work? A circular polarizer would probably be less effective.

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Art_P
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My guess...
In reply to Anders W, Mar 1, 2013

#2: linear polarizer?

#3: shifted WB or other in camera adjustment to reduce sensitivity to the purple? (I see the flare is still present, but not blown out and less objectionable)

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Anders W
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Re: Purple flare: Causes and remedies
In reply to MAubrey, Mar 1, 2013

MAubrey wrote:

Anders W wrote:

MAubrey wrote:

Anders W wrote:

Macx wrote:

Well, I suppose it isn't a lens hood.

Pol-filter perhaps?

Yes, in one case. Which? And for what reason can a polarizer remove lens flare and/or its purpleness?

Hmmm. So at least one answer is DIY filters for the 7-14mm...

Yes or the WonderPana from Fotodiox.

http://www.dpreview.com/news/2012/12/06/Fotodiox-launches-WonderPana-Filter-Systems-145-66

However, I wouldn't jump to the conclusion that a polarizer always helps as well as it did in this particular case.

Don't worry. I always prefer walking to conclusions rather than jumping. In this case, I'm most definitely awaiting for the full unveiling.

Good. I thought so. Merely a safety precaution.

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s_grins
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Re: Purple flare: Causes and remedies
In reply to Anders W, Mar 1, 2013

In my understanding, it is not a particular lens, but all wide lenses with aspheric element are prone to purple flare. Aspheric element works like a Fresnel prism that splits light spectrum more aggressively than normal spherical glass. Red light with longer wave length refracts more than blue light.

How to fight this purple flare? As others said: use lens hood.



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Timur Born
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Re: Purple flare: Causes and remedies
In reply to Anders W, Mar 1, 2013

Shot as Jpg or Raw? Tone curves involved?

Pol filter was my guess for no. 2. On no. 3 I notice some increased noise or maybe just JPG artifacts and the file is Bigger in size.

Btw, either you moved the cam or cropped differently in one image.

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Anders W
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Re: Purple flare: Causes and remedies
In reply to Macx, Mar 1, 2013

Macx wrote:

Anders W wrote:

Macx wrote:

Anders W wrote:

Macx wrote:

Well, I suppose it isn't a lens hood.

No.

Pol-filter perhaps?

Yes, in one case. Which? And for what reason can a polarizer remove lens flare and/or its purpleness?

2nd one? Well, the flare is a reflection and as such polarised light? *guessing*

It's image two of three, yes. And yes, you are on the right track. Could you develop your thought a bit and what kind of polarizer is required in this case?

Well, since it's reflections from light that's already passed through the filter, a simple filter like the ones in polaroid sunglasses could work? A circular polarizer would probably be less effective.

Right again (I think). I used a linear polarizer. A circular probably wouldn't work in this case (though truth be told, I didn't even try so I will).

We all know that reflected light is polarized. What that means is that only light polarized in a certain way will be reflected in the first place. The idea here is to turn the polarizer so that it lets through only light polarized in such a way that the troublesome reflections later in the optical path do not occur.

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Anders W
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Re: Purple flare: Causes and remedies
In reply to Sanpaku, Mar 1, 2013

Sanpaku wrote:

Yes, I remember we discussed that possibility. But that (reflections off the sensor, back on the rear element, and then back again) is not what I think is going on. Note that didn't use the 7-14 in this case. Also note that if you inspect the E-M5 sensor with your eyes, the reflections you see are redish and greenish, not purple (no real sign of blue).

It occurred to me that if one used a linear polarizer on the lens, it would automatically reduce reflections (from certain angles) off the sensor. Is that what is in your images?

As already pointed out, the second of the three images is shot with a linear polarizer. And yes, the effect is to reduce reflections later in the optical path although not necessarily those off the sensor (only or primarily).

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Anders W
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Re: My guess...
In reply to Art_P, Mar 1, 2013

Art_P wrote:

#2: linear polarizer?

Yes, Macx has already guessed that correctly.

#3: shifted WB or other in camera adjustment to reduce sensitivity to the purple? (I see the flare is still present, but not blown out and less objectionable)

A mere shift of WB would shift the WB everywhere in the image. But the ceiling looks the same as far as WB is concerned. It's only the color of the flare that differs.

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Anders W
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Re: Purple flare: Causes and remedies
In reply to s_grins, Mar 1, 2013

s_grins wrote:

In my understanding, it is not a particular lens, but all wide lenses with aspheric element are prone to purple flare. Aspheric element works like a Fresnel prism that splits light spectrum more aggressively than normal spherical glass. Red light with longer wave length refracts more than blue light.

Interesting idea. But in this case I used a short tele (45/1.8) without any aspheric elements.

How to fight this purple flare? As others said: use lens hood.



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Anders W
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Re: Purple flare: Causes and remedies
In reply to Timur Born, Mar 1, 2013

Timur Born wrote:

Shot as Jpg or Raw? Tone curves involved?

RAW. Processed in LR. No tone curves involved.

Pol filter was my guess for no. 2.

Yes. Macx already guessed that and it is correct. A linear polarizer to be precise.

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.

Btw, either you moved the cam or cropped differently in one image.

I might have inadvertently moved things very slightly before capturing image three. I removed the card inbetween. But this is immaterial to the outcome.

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Re: Purple flare: Causes and remedies
In reply to Anders W, Mar 1, 2013

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?

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Anders W
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Re: Purple flare: Causes and remedies
In reply to Bizzarrini, Mar 1, 2013

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).

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