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09-28-2010, 04:28 AM   #1
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K-7 Aurora long time exposure gives rings in the picture

Hi,
I took some shoots last night of aurora borealis.

If you have a look at the picture below you will see some nasty rings in the centre of the picture. Whats the reason for that and how to avoid this in the future?

Thanks a lot,
Mace

Picture infos:
Camera K-7
Lens SMC DA 18-55 at 18mm
f5.6
iso 400
30sec
0ev

Rawformat DNG
opened with Aperature Software

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File Type: zip Matze - 000430 - 2010-09-27.jpg.zip (643.8 KB, 107 views)
09-28-2010, 04:33 AM   #2
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Maybe its heat related
Do you get them in normal shots ?
09-28-2010, 04:42 AM   #3
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Temperature was only -2C. Usually I don't get them in normal shots at these temperatures.
09-28-2010, 04:42 AM   #4
Col
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Try googling: "colour banding in sky" or if you insist: "color banding in sky". Generally related to converting to 8bit jpg... just not enough subtlety for sky shades. You should see my early LCD TV!

09-28-2010, 04:51 AM   #5
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The color banding is not the problem. I have the same effect in the raw DNG file.

It looks more to me like a radial lens disturbance. It has definitely to do with the lens or the sensor.
09-28-2010, 04:52 AM   #6
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If you check the zipped file and zoom to the center you will see what I mean.
09-28-2010, 05:04 AM   #7
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Beginning to think its a "rounding" beat pattern from the dark frame subtraction.
09-28-2010, 05:15 AM - 1 Like   #8
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I had this same problem long time ago with Minolta D7Hi. In the end I found that it was the UV-filter which caused those rings. So if you are using a filter, try removing it first.

09-28-2010, 05:28 AM   #9
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hm, I am indeed using a skylight filter for lens protection. That could be an explanation.
09-28-2010, 05:31 AM - 1 Like   #10
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I agree with Nitrok, those look like newton rings. Do you have a filter/polariser on the lens? I recommend that you take it off.
09-28-2010, 05:40 AM   #11
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Still like the image.

QuoteOriginally posted by Mace Quote
hm, I am indeed using a skylight filter for lens protection. That could be an explanation.
Simple (and cheapest) solution! I don't use "clear" filters for protection so never crossed my mind. I don't know about your country but here a decent filter costs more than the kit lens! (Poor under appreciated glass)

Last edited by Col; 09-28-2010 at 05:47 AM.
09-28-2010, 05:42 AM   #12
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Useful thing to learn. Thanks.
09-28-2010, 05:59 AM   #13
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Try using a different RAW Converter, sometimes images get interpreted differently.
09-28-2010, 06:09 AM   #14
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QuoteOriginally posted by thazooo Quote
Try using a different RAW Converter, sometimes images get interpreted differently.
i'm doubtful of that, Newton rings are a known optical phenomenon. To suggest that the demosaicing algorithm produced an artefact that looks exactly like one is very far-fetched.
09-28-2010, 06:10 AM   #15
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Its the same in every RAW converter.

The filter seems to be the most reasonable explanation of the problem, unless the lens itself causes it.

I guess I'll do my longterm exposures in future without a filter.
Still, living in a very rough, rocky and dusty environment, I prefer to have a protection filter on my lens on a regular basis.
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