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Why is there a purple blur around fire in photos? I'm using a Sony Xperia phone.

example

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I've run into that when taking pictures of Raku ceramics in and just out of the kiln. I think it's a heavy dose of near IR overwhelming (or at least leaking through) the camera's IR blocking filter. With a phone this may be more of a problem than with a high end SLR- the IR block filter may not be as efficient.

If it -is- IR then whatever lens coatings the phone camera may have are probably ineffective, leading to enhanced flare and the effect you see in your photos.

  • What IR gets past the cameras filter will be registered more or less equally by all three color channels. Near IR would be much stronger in the red channel, moderately stronger in the green channel, and very weak in the blue channel. – Michael C Aug 10 '18 at 0:26
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Fire can definitely be photographed without such flare.

enter image description here
EOS 5D Mark II, EF 24-105mm f/4L IS, ISO 1600, f/5.6, 1/1250 sec. Color Temp. 2500K, +1.33 Exposure and other minor adjustments in post.

Even when the flames are grossly overexposed, such an image does not necessarily have to demonstrate the purple halo seen in the example photo in the question.

enter image description here
EOS 5D Mark II, EF 24-105mm f/4L IS, ISO 6400, f/4, 1/60 sec. Color Temp. 2800K, -1.50 Exposure and other adjustments in post.

As others have said, the camera you are using is contributing to the purple halo. It could be something as simple as smudges on the front of the lens, as is often demonstrated by photos taken with phone cameras. Or it could be related to the high amount of infrared energy being emitted by the fire.

One would expect such large amounts of IR to show up more equally in all three color channels, though. One of the main reason more sophisticated cameras have IR filters in the sensor stack is because the sensels under each of the three color filters in bayer masked sensors are all more or less equally sensitive to wavelengths above 800 nanometers.

enter image description here

If the halo were being caused by poor IR filtering, it would probably show up more as a grey/white flare than a purple one. I think the color of the halo is much more likely to be indicative of the color of your camera's lens coatings. Such colors are often a characteristic of lens flare.

  • I agree mostly with what you've said. My experience with the raku kiln was with a D800 which seems to have a decent IR cut filter, but still had a definite purple cast to the images. Of course kiln temperatures are usually much higher than campfire/bonfire temperatures. I'm less confident that a cell phone camera has an equivalently effective IR cut filter- I can see a faint glow from an IR remote with my iPhone camera. – BobT Aug 9 '18 at 17:30
  • @BobT I doubt it is IR you are observing. What gets past the IR filter would be registered by all three color channels almost equally. Most remotes are near-IR and register much more red and green than blue when photographed by 'regular' cameras. – Michael C Aug 10 '18 at 0:24
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Common lens flare do to either a dirty lens or poor lens coatings (or lack thereof).

  • Or reflections off the inside of the lens barrel. To me, this looks more like reflections off something to either side of the lens, but it's hard to say for sure. – dgatwood Aug 10 '18 at 20:42

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