Purple flare: Causes and remedies

Started Mar 1, 2013 | Discussions thread
Anders W
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Re: What exactly are you doing with 2nd and 3rd pictures???
In reply to teeranui, Mar 3, 2013

teeranui wrote:

Anders W wrote:

There have been quite a bit of discussion here over the last half a year or so about the fact that the E-M5 (and possibly some other MFT cameras too) tend to produce purple flare in certain situations. The problem is particularly noticeable with the Panasonic 7-14 but that is, in all likelihood, merely a consequence of the fact that the 7-14, like other UWAs, is more prone to flare in general than other lenses. The tendency for the flare to go purple is there with any lens, not just the 7-14.

Ever since the first major thread on the subject about half a year ago (see here) I have been trying to figure out exactly what is going on here. Why is the flare, which is neutral white/gray with other cameras (e.g., the G1) going purple on the E-M5 (and perhaps some other MFT bodies as well)?

I have had what I consider to be reasonably promising ideas about that for a while now but I have been waiting for some key equipment that would allow me to test my ideas to a somewhat greater extent before spelling them out. That equipment finally arrived so here goes.

As a testing stage, I chose my kitchen ceiling with associated spotlights. As previously observed, strong lights just outside the frame is one of several conditions that might cause the dreaded purple flare to appear and as shown by the first of the three images posted below (with the spotlights just outside the top of the frame), I had no difficulties reproducing the problem with the 45/1.8 on the E-M5.

What we see here, by the way, is in all likelihood the same phenomenon that has received so much attention with regard to the iPhone 5. Just google the phrase "purple flare" without specification, e.g., by means of this link and you will see what I mean.

Apple's official explanation (see here) is the following:

"Most small cameras, including those in every generation of iPhone, may exhibit some form of flare at the edge of the frame when capturing an image with out-of-scene light sources. This can happen when a light source is positioned at an angle (usually just outside the field of view) so that it causes a reflection off the surfaces inside the camera module and onto the camera sensor. Moving the camera slightly to change the position at which the bright light is entering the lens, or shielding the lens with your hand, should minimize or eliminate the effect."

Well, so far so good, but we all kind of knew that already. The key question is why the flare now goes purple whereas it previously stayed neutrally colored (white/gray) and, as far as I can tell, not much of a convincing answer has been provided yet, by Apple or anyone else. As you can see here, DPR, in their "quick review", suggest a lot of potential explanations but reject most of them and in the end have little more to offer than speculations about the new saphire glass in front of the lens and the lens coatings.

So what's really going on here? In the second and third image posted below, the flare and/or its purpleness has been vastly reduced. So what did I do to achieve that?

No, I didn't change the light, the camera position, the framing, the lens, the focus or the aperture. The light remained unchanged and the camera was firmly fixed on a tripod. All images were shot with the E-M5 and the 45/1.8 at f/1.8 and 1/5 s. And no, I didn't do anything to reduce the flare or its purpleness in PP. What I did to reduce it was done at the time of shooting. So what did I do? Two different solutions were tried, one for image two and another for image three.



What exactly are you doing with 2nd and 3rd pictures?

For the second image, I used a linear polarizer. For the third image, I used heavy purple filtering (CC30M + CC70M + 80A). The polarizer works because the purple flare results from reflection later in the optical path and the polarizer eliminates the light that would otherwise have been reflected. For the impact of the purple filtering, see here:

http://forums.dpreview.com/forums/post/50958004

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