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CHDK 1.4 and Powershot SD900: Blue Flare during Long Exposures

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Offline OlDoinyo

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CHDK 1.4 and Powershot SD900: Blue Flare during Long Exposures
« on: 21 / September / 2015, 11:57:33 »
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This may not be a question about CHDK as such, but rather about what happened when it interacted with a particular camera. With 60-second exposures, flare-like artifacts appear in  the left corners but not in the right corners This can be seen both in the image and in the dark frame:



Subtracting the dark frame removes most of this artifact, but it is severe enough to be a major nuisance, as some degradation of image quality at the corners is noticeable, and it were better to solve the underlying problem.

-CHDK version was 1.4.0.

-The script used was meteor2.lua; thie frame was part of a series of 40.

-The ISO was 200 and the aperture was set to f/2.8 with the ND out.

-The scene was much dimmer than the top picture suggests; the foreground was almost invisible without a flashlight.

-The display backlight illuminated briefly between frames; otherwise, the only light on the camera was a flashing orange LED on the rear and a blue LED which illuminated briefly between frames.

-The effect does not appear when operating the camera in the usual single-shot mode, even at ISO 1600.

-What is this?
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Offline waterwingz

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Re: CHDK 1.4 and Powershot SD900: Blue Flare during Long Exposures
« Reply #1 on: 21 / September / 2015, 12:19:35 »
Quote from: OlDoinyo on 21 / September / 2015, 11:57:33
What is this?
google for  "amp glow"

or search this forum for it
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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline OlDoinyo

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Re: CHDK 1.4 and Powershot SD900: Blue Flare during Long Exposures
« Reply #2 on: 21 / September / 2015, 21:48:08 »
From what I hear, there is not much to be done about the problem, then, other than to live with it as best we may.
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Offline waterwingz

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Re: CHDK 1.4 and Powershot SD900: Blue Flare during Long Exposures
« Reply #3 on: 21 / September / 2015, 22:33:52 »
Quote from: OlDoinyo on 21 / September / 2015, 21:48:08
From what I hear, there is not much to be done about the problem, then, other than to live with it as best we may.
That's pretty much it.

Some people use shortedr exposures and then stack the images in post processing.  I don't know if that's better than using dark frame subtraction.  YMMV.

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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline reyalp

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Re: CHDK 1.4 and Powershot SD900: Blue Flare during Long Exposures
« Reply #4 on: 21 / September / 2015, 22:34:12 »
Quote from: OlDoinyo on 21 / September / 2015, 21:48:08
From what I hear, there is not much to be done about the problem, then, other than to live with it as best we may.
If you use a dark frame, you can remove a lot of  it. See http://chdk.setepontos.com/index.php?topic=12295.msg121437#msg121437

edit:
Oops, I missed that you had actually used dark frame, and still see a reduction in quality. If the amp glow takes up a substantial part of sensor range, this is inevitable. Some things that might help get better results

- Use raw, if you aren't already
- Make sure your darks are taken at a similar sensor temperature to the actual shots.
- Make a dark frame that is an average or median of several dark frames.

Finally, if you are feeling brave, it may be possible to address the underlying cause with hardware mods http://chdk.setepontos.com/index.php?topic=8949.0
« Last Edit: 21 / September / 2015, 22:46:17 by reyalp »
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Offline OlDoinyo

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Re: CHDK 1.4 and Powershot SD900: Blue Flare during Long Exposures
« Reply #5 on: 21 / September / 2015, 23:27:10 »
I did try the Raw save option for the sequence in question, but the resulting files looked laughably bad--far worse than the Jpegs, and they increased the interframe  cycle time as well, definitely not a good thing. I don't know what I did wrong there.

The hardware hack is far beyond my ability to attempt.

The final stacked result of the sequence, using DFS and partially cropping off the left of the frame, was this:



Even with cropping, one can see increased noise and wacky color at the left corners.

PS the Captchas on this forum are really annoying!



« Last Edit: 21 / September / 2015, 23:29:00 by OlDoinyo »
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Offline waterwingz

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Re: CHDK 1.4 and Powershot SD900: Blue Flare during Long Exposures
« Reply #6 on: 21 / September / 2015, 23:36:39 »
The captcha go away after you have posted as few more times.  A small price to pay for no spam these days.  It used to be quite bad.
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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline reyalp

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Re: CHDK 1.4 and Powershot SD900: Blue Flare during Long Exposures
« Reply #7 on: 22 / September / 2015, 00:04:38 »
Quote from: OlDoinyo on 21 / September / 2015, 23:27:10
I did try the Raw save option for the sequence in question, but the resulting files looked laughably bad--far worse than the Jpegs, and they increased the interframe  cycle time as well, definitely not a good thing.
You should be able to get significantly better results with raw: In the jpeg, the DR has been reduced, and the signal has been irreversibly mixed with the amp glow and noise.

However, you need to find the right tools and workflow to get good results. For regular snapshots raw often isn't worth effort, but for cases like this it can make a big difference.

From the thread I linked earlier, here's a 64 sec exposure from D10, camera jpeg
https://app.box.com/s/eum5n5smdjj8or7h5k3s8j6a3zma5lol
and jpeg from processing raw in rawtherapee with dark frame subtracted
https://app.box.com/s/74epoa5y3fd7ter8fs6gj6zmpkf0guoe

Of course using raw will reduce your shooting speed a lot, that can't be avoided.

Another alternative is to get a newer camera. As I mentioned in that other thread, cameras from 2012 on (and perhaps earlier) don't seem to suffer from amp glow nearly as much. A $50 low end refurb might be the easy answer.
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