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Light bleeding from display to sensor? --> probably Amp glow (SX200IS)

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Offline Darkness

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Light bleeding from display to sensor? --> probably Amp glow (SX200IS)
« on: 27 / October / 2012, 18:05:55 »
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I recently used CHDK to take some nice night sky pictures using the long exposure override.

However, there is something strange going on with exposures that are 2-3 minutes or longer.
Example image:
http://www.flickr.com/photos/55760207@N03/8057785600/#in/photostream

Notice the light in the top left corner.
The only lightsource that I can think of during the capture would be the display.

Did any of you notice similar effects when using extra long exposures?
Could this be light reflected from the display on the rock below (tiny tripod) or display backlight escaping inside the camera to the corner of the sensor?
« Last Edit: 27 / October / 2012, 20:40:06 by Darkness »
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Offline waterwingz

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Re: Light bleeding from display to sensor? (SX200IS)
« Reply #1 on: 27 / October / 2012, 18:17:38 »
Its called amp glow

https://www.google.com/search?q=amp+glow


You use dark frame subtraction to correct it either in the camera or during post processing.
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Offline srsa_4c

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Re: Light bleeding from display to sensor? (SX200IS)
« Reply #2 on: 27 / October / 2012, 18:37:25 »
Quote from: waterwingz on 27 / October / 2012, 18:17:38
Its called amp glow
It could also be what radioman193 found (unfortunately all pictures in his thread disappeared): http://chdk.setepontos.com/index.php?topic=7583.0
... which is slightly different from amp glow, as the noise he successfully shielded is not from the sensor.
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Offline waterwingz

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Re: Light bleeding from display to sensor? (SX200IS)
« Reply #3 on: 27 / October / 2012, 18:52:26 »
Quote from: srsa_4c on 27 / October / 2012, 18:37:25
Quote from: waterwingz on 27 / October / 2012, 18:17:38
Its called amp glow
It could also be what radioman193 found (unfortunately all pictures in his thread disappeared): http://chdk.setepontos.com/index.php?topic=7583.0
... which is slightly different from amp glow, as the noise he successfully shielded his not from the sensor.

Ralph is still pretty active on Facebook - he basically swamps the CHDK group page with is images every day.   I'll ask him if he can update the images in his post.

« Last Edit: 27 / October / 2012, 18:55:37 by waterwingz »
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Offline Darkness

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Re: Light bleeding from display to sensor? (SX200IS)
« Reply #4 on: 27 / October / 2012, 19:46:57 »
thanks for the replies so far. I didn't know about amp glow. Looks like that might be it.

Next time I will do some tests with letting things cool down in between shots and check the ccd temperature a bit to see if anything is influencing the amount of glow.

It would be nice if Ralph can fix the images in that thread. That looks like something I could try.
The glow always has the same shape and line pattern, but in varying intensity. I do have 2 pictures that were taken sequentially with the same exposure and the second one does have more glow.

Any way to remove/reduce it with software?
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Offline waterwingz

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Re: Light bleeding from display to sensor? (SX200IS)
« Reply #5 on: 27 / October / 2012, 20:17:00 »
Quote from: Darkness on 27 / October / 2012, 19:46:57
Any way to remove/reduce it with software?
With long exposures (more that a couple of seconds)  your camera should automatically take a second picture after every exposure with the shutter closed.  It then automatically subtracts this photo from the first one.  This is called "dark frame subtraction".    As this essentially doubles the time to take each exposure,  CHDK allows you to turn this off (on the assumption you will post process the darkframe out).   To do this,  you need to periodically cover the lens and take a shot of darkness.

The menu option in the  RAW menu called  "Dark Frame Subtraction" determines whether the camera uses the internal process or not.  Its active regardless of whether you have RAW enabled or not.

If its set to On or Auto you should see a reduction in amp glow in you pictures.

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Offline Darkness

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Re: Light bleeding from display to sensor? (SX200IS)
« Reply #6 on: 27 / October / 2012, 20:39:15 »
It is set to auto. I didn't know that this option was causing the double exposure time, but the picture above and every other picture with similar glow have been taken with Dark frame subtraction. I guess turning it of will only make things worse :)
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Offline waterwingz

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Re: Light bleeding from display to sensor? --> probably Amp glow (SX200IS)
« Reply #7 on: 27 / October / 2012, 20:46:15 »
Quote from: Darkness on 27 / October / 2012, 20:39:15
It is set to auto. I didn't know that this option was causing the double exposure time, but the picture above and every other picture with similar glow have been taken with Dark frame subtraction. I guess turning it of will only make things worse :)
Try it and see.   Its possible that's actual light leakage in your camera - which might not show up on the dark frame and thus not get cancelled out.

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Offline reyalp

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Re: Light bleeding from display to sensor? --> probably Amp glow (SX200IS)
« Reply #8 on: 27 / October / 2012, 21:02:29 »
Quote from: waterwingz on 27 / October / 2012, 20:46:15
Quote from: Darkness on 27 / October / 2012, 20:39:15
It is set to auto. I didn't know that this option was causing the double exposure time, but the picture above and every other picture with similar glow have been taken with Dark frame subtraction. I guess turning it of will only make things worse :)
Try it and see.   Its possible that's actual light leakage in your camera - which might not show up on the dark frame and thus not get cancelled out.
I've definitely seen the "amp glow" effect with dark frame enabled. Not sure why the dark frame doesn't get more of it.
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Offline outslider

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Re: Light bleeding from display to sensor? --> probably Amp glow (SX200IS)
« Reply #9 on: 28 / October / 2012, 03:56:00 »
My sx130is has the same problem and the DFS also doesn't correct it much. And I have also this in top-left corner and the same color of the glow.
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