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11-15-2011, 07:13 AM   #1
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645D hot pixel mapping in Bulb mode?

Last week I did some nighttime photography in B mode, with exposures between 1 minute and 15 minutes. As exposure times grew longer, the number of hot/warm and dead/cold pixels increased.

Does anyone know if it is supposed to happen this way, that the camera dark-frame noise-reduction becomes less effective at long exposures? If so, I wonder how many bad pixels can be considered acceptable. Cloning them out manually is a recipe for madness...

Also, I noticed that after going through a pixel mapping, the camera did not return to normal operation. The LCD panel displayed "0s nr", in the same way as it displays remaining time for dark-frame subtraction after a B exposure. It remained this way until turned off and on again, or until meter operation timed out and reactivated by half-pressing the shutter.

The pixel mapping had no effect whatsoever on the occurrence of bad pixels in B mode. Same pixels, same hotness/coldness.

Anyone having similar experiences?

Thanks,
Jostein

PS firmware version: 1.11.

11-15-2011, 08:03 AM   #2
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Jostein, can you post an example of what you are talking about? I have done long night exposures with the 645D and noise increases with exposure. I can at least confirm if I am getting similar results.
11-15-2011, 08:06 AM   #3
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On the K-x, the dead pixel mapping is kind of blocked when the sensor is a bit hot, to not consider some hot pixel dead. i guess that 645D is acting the same way.

If you shoot raw, those hot pixel can be easily treated with some noise color reduction in any decent software of PP.
11-15-2011, 08:36 AM   #4
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Try enabling slow shutter speed noise reduction (DFS). That setting should fix your problems. The issue is likely not being caused by hot pixels but rather noise.


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11-15-2011, 09:43 AM   #5
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QuoteOriginally posted by Adam Quote
Try enabling slow shutter speed noise reduction (DFS). That setting should fix your problems. The issue is likely not being caused by hot pixels but rather noise.
DFS *was* enabled. Thought I was explicit about that...
11-15-2011, 09:44 AM   #6
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QuoteOriginally posted by Yamanobori Quote
Jostein, can you post an example of what you are talking about? I have done long night exposures with the 645D and noise increases with exposure. I can at least confirm if I am getting similar results.
Thanks Yamanobori. Pics coming in a couple of hours.
11-15-2011, 10:28 AM   #7
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QuoteOriginally posted by Adam Quote
Try enabling slow shutter speed noise reduction (DFS). That setting should fix your problems. The issue is likely not being caused by hot pixels but rather noise.
In the case of my 645D switching it off has no bearing as the camera does it anyway! I also get a handful of hot or dead pixels in long long expposures
11-17-2011, 02:29 AM   #8
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Here are two images showing what I'm concerned about, overview and detail. The detail show a number of bright spots. For the second image, I also have a version exposed for 30 seconds at ISO 1600, and while the general noise level is higher, there are fewer bright spots on high ISO. Any suggestions for rational explanation will be much appreciated...

The first is a 487s (8 min, 7 s) exposure at ISO 200:
Name:  645-20111107-0034-PF.jpg
Views: 1020
Size:  218.3 KB

Detail shows a number of bright spots, but also what looks like luminescent dust. Beats me how this emerge...
Name:  645-20111107-0034-PF-2.jpg
Views: 905
Size:  283.6 KB

Second image is a 916 seconds (15 min, 16 s) exposure at ISO 200:
Name:  20111109-0074-PF.jpg
Views: 982
Size:  163.4 KB

In which the same phenomenon occurs:
Name:  20111109-0074-PF-2.jpg
Views: 953
Size:  286.2 KB

11-19-2011, 08:43 PM - 1 Like   #9
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I think you are seeing noise from long exposures. While I have not done anything over 5 minutes to date, I have had similar artifacts; both black and white. Ambient temperatures will also add to these--the warmer the night, the greater the noise. It is a bit of a pain, but at least you don't have to worry about reciprocity...

The only thing I can think of to cut down on retouching would be to use an astrophotography stacking program and take two or more exposures and use the programs to eliminate random noise.
11-20-2011, 07:18 AM   #10
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QuoteOriginally posted by Yamanobori Quote
The only thing I can think of to cut down on retouching would be to use an astrophotography stacking program and take two or more exposures and use the programs to eliminate random noise.
Thanks Yamanobori!

The first thing this phenomenon made me think of was a scientific image I once saw, of how cosmic rays could be detected in a fog chamber...

Today I did some 15 min. exposures at ISO 200 with the lens cap on. The number of hot pixels are roughly the same in consecutive exposures, but it's never the same pixels. I guess that's why the dark-framing doesn't catch it in the first place. No pixels are permanently stuck.

I also did the same with the K-5. There are a few hot pixels here too, but a lot fewer per megapixel than with the 645D.

I will indeed try to play with some astrophotography stacking program.

Thanks again.
07-10-2012, 08:48 PM   #11
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Maybe somehow it is dust.
07-11-2012, 03:52 AM   #12
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QuoteOriginally posted by alunfoto Quote
Here are two images showing what I'm concerned about, overview and detail. The detail show a number of bright spots. For the second image, I also have a version exposed for 30 seconds at ISO 1600, and while the general noise level is higher, there are fewer bright spots on high ISO. Any suggestions for rational explanation will be much appreciated...

The first is a 487s (8 min, 7 s) exposure at ISO 200:
Attachment 109280

Detail shows a number of bright spots, but also what looks like luminescent dust. Beats me how this emerge...
Attachment 109281

Second image is a 916 seconds (15 min, 16 s) exposure at ISO 200:
Attachment 109282

In which the same phenomenon occurs:
Attachment 109283
I have had these too... in the bottom left corner


07-11-2012, 06:36 AM   #13
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Spurious noise

QuoteOriginally posted by Gryphen Quote
Maybe somehow it is dust.
Not at all, Gryphen.
It's kinda weird for us ordinary mortals, though...
Turns out this is a phenomenon well known among astrophotographers. They call it "Spurious noise", and is actually caused by backscattering and decay products from occasional cosmic rays slipping through the atmosphere.

I wrote something about it over at my own blog soapbox, but I'll save you the drivel.
Here's a link to a scientific paper in PDF describing the issue very accurately, written by astronomer Don Groom at Lawrence Berkeley National Laboratory, USA.
11-15-2012, 02:43 AM   #14
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guys, i just started noticing red sharp stripe *hot stripe on my 645D,when shooting 400iso and above...any ideas. here is cropped detail. tried mapping, turning NR on and off...it goes not entirely over the length of sensor but most of it
Attached Images
 
11-15-2012, 04:52 AM   #15
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I had that stripe problem too. Sent the camera back to Pentax - sensor issue - they fixed it. But expect it to go back to Japan, and to have no camera for a while :-(
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