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08-16-2009, 04:27 AM   #1
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Does my sensor have a problem?

After checking out and editing some night sky photos I took last night with my K-m I noticed that on some pictures there is a reddish/violet hue on the upper border. As the photos were shot in different directions I can exclude that it is due to an external light artefact. Also there is no LED or lamp whatsover on the camera that would explain this to me. The problem typically occured at exposures ranging from 50-80 seconds at ISO 200-400 (didn't turn sensitivity up any higher) and became distinctively visible after post processing in which I usually pushed exposure by 4.0 stops, increased blacks slightly and cranked up contrast to the max in Camera Raw 4.6. Note that I had never done any PP before so my methods may seem a bit amateurish (the results look fine to me anyway).

So my question is: is there something wrong with my sensor or is this just the price I pay for doing several >1 min exposures in a row with an entry level camera and then pushing exposures? I remember there being a similar thread recently but don't recall what the answer to it was, nor can I seem to find it.

Also if this is not a defect, can somebody please tell me how to make this colour hue less visible in Photoshop? My pictures generally turned out pretty well and usable overall and I wouldn't mind correcting this artefact to make them a bit more attractive.

Here's an example shot at ISO 200, 10mm, F/3.5, 78 sec exposure:

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08-16-2009, 04:41 AM   #2
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Try taking a "black" long exposure with the lens cap on.
If its a sensor problem or at least some camera circuitry problem, you should see the same fault appearing in the completely dark exposure.
08-16-2009, 05:11 AM   #3
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Did that and the hue is there. It really becomes visible particularly after pushing exposures.
08-16-2009, 05:57 AM   #4
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QuoteOriginally posted by amaurosis Quote
.......
Also if this is not a defect, can somebody please tell me how to make this colour hue less visible in Photoshop? .......
Here's an example shot at ISO 200, 10mm, F/3.5, 78 sec exposure:
If I'm not mistaken this is inherent with the sensor on long exposures.

If you take a dark frame after your regular shot, you can import this as a separate layer in Photoshop and subtract out the noise. You might have to play around a bit with the Opacity setting.
I found a simple write up on the subject below.
Dark Frame Subtraction using Adobe Photoshop by Chris Limone

08-16-2009, 07:47 AM   #5
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This issue of purple patches has turned up in a few other threads - eg

https://www.pentaxforums.com/forums/pentax-dslr-discussion/67165-strange-ligh...exposures.html

It is indeed [CCD?] sensor related, as kittykat suggests, something to do with the inherent noise underlying the image laid down by the sensor. The thread above has some good discussion and some suggested solutions.
08-16-2009, 10:33 AM   #6
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QuoteOriginally posted by amaurosis Quote
After checking out and editing some night sky photos I took last night with my K-m I noticed that on some pictures there is a reddish/violet hue on the upper border. As the photos were shot in different directions I can exclude that it is due to an external light artefact. Also there is no LED or lamp whatsover on the camera that would explain this to me. The problem typically occured at exposures ranging from 50-80 seconds at ISO 200-400 (didn't turn sensitivity up any higher) and became distinctively visible after post processing in which I usually pushed exposure by 4.0 stops, increased blacks slightly and cranked up contrast to the max in Camera Raw 4.6. Note that I had never done any PP before so my methods may seem a bit amateurish (the results look fine to me anyway).

So my question is: is there something wrong with my sensor or is this just the price I pay for doing several >1 min exposures in a row with an entry level camera and then pushing exposures? I remember there being a similar thread recently but don't recall what the answer to it was, nor can I seem to find it.

Also if this is not a defect, can somebody please tell me how to make this colour hue less visible in Photoshop? My pictures generally turned out pretty well and usable overall and I wouldn't mind correcting this artefact to make them a bit more attractive.

Here's an example shot at ISO 200, 10mm, F/3.5, 78 sec exposure:
Amp glow. Fairly common in all cameras w/ long exposures. Should be eliminated (or significantly reduced) w/ in-camera dark frame subtraction.
Flickr: Discussing What's your experience with DSLR amp glow on 2 min+ exposures? in Long Exposure



Oddly enough there seems to be 3 "heating" components producing the effect w/ the major one being right where you see it.
No defect (per se) just is what it is and why they have "dark frame subtraction".....
08-16-2009, 02:01 PM   #7
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QuoteOriginally posted by rawr Quote
This issue of purple patches has turned up in a few other threads - eg

https://www.pentaxforums.com/forums/pentax-dslr-discussion/67165-strange-ligh...exposures.html

It is indeed [CCD?] sensor related, as kittykat suggests, something to do with the inherent noise underlying the image laid down by the sensor. The thread above has some good discussion and some suggested solutions.
That's the thread I was looking for.

Thanks, guys. Great help as always
08-17-2009, 05:31 AM   #8
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As others already wrote it is normal due to sensor heating. Just enable "long exposure noise reduction", it is in custom settings.

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