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02-11-2018, 08:52 AM   #1
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Strange grid/moire pattern at high ISO, is this normal?

I was really pushing the low end of my 645Z the other night while shooting the bioluminescent algae under moonless skies. In reviewing the images, I noticed this unusual pattern in nearly all of the images at ISO 4000 and higher. Compounding the effect, they were all long exposures, from 3-7 minutes.

I don't expect to see this again unless I'm really maxing out the detection limits of the sensor, but want to make sure this is an expected artifact and not some damage to my sensor, etc.

All good?

---------- Post added 02-11-2018 at 07:59 AM ----------

In the final processed image you can make out the pattern quite readily:

Bioluminescent Algae at Big Sur

645Z & DFA 35/3.5
7 minutes @ ISO 4000




____________________

To enhance the effect for inspection purposes, this attachment is a stack of 11 successive 10sec dark images, combined using "standard deviation" blend mode.

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PENTAX 645Z  Photo 

Last edited by mikeSF; 02-11-2018 at 09:01 AM.
02-11-2018, 09:13 AM - 1 Like   #2
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Could be a light leak trough the viewfinder. I had problems with strange patterns appearing during long exposures with my K-3 when I didnít shield the viewfinder.
02-11-2018, 09:15 AM - 1 Like   #3
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Is it always in the dead centre of the image, Mike? Any filters in front of the lens? Happening with just one, or more than one lens?
02-11-2018, 09:18 AM - 6 Likes   #4
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That looks like a resampling artifact related either to lens distortion correction or stacking.

These kinds of artifacts occur with high-frequency noise or grain and resampling because there will be an undulating geometric pattern in the image where some parts of the image are being shifted by an exact number of pixels (which preserves high frequency detail/noise/grain) and some parts will be shifted by fractions of a pixel (which tend to attenuate the detail/noise/grain). Any subsequent processing of the image light levels (boosting shadows, manipulating the curves, gamma, etc.) will convert that pattern of preserved versus resampled grain into a pattern of lighter and dark bands like this.

The effect may also be exaggerated by an underlying checkerboard noise artifact in the sensor (that's been there from day 1 or has developed over time) or it may be something in the demosaicer.

Does the artifact show up in single, unprocessed, uncorrected RAW images? What if you develop the RAW image in a different piece of software?

02-11-2018, 09:47 AM - 1 Like   #5
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QuoteOriginally posted by Fries Quote
Could be a light leak trough the viewfinder. I had problems with strange patterns appearing during long exposures with my K-3 when I didnít shield the viewfinder.
viewfinder was taped over. no extraneous light of any type was present. just the stars

---------- Post added 02-11-2018 at 08:49 AM ----------

QuoteOriginally posted by photoptimist Quote
That looks like a resampling artifact related either to lens distortion correction or stacking.

These kinds of artifacts occur with high-frequency noise or grain and resampling because there will be an undulating geometric pattern in the image where some parts of the image are being shifted by an exact number of pixels (which preserves high frequency detail/noise/grain) and some parts will be shifted by fractions of a pixel (which tend to attenuate the detail/noise/grain). Any subsequent processing of the image light levels (boosting shadows, manipulating the curves, gamma, etc.) will convert that pattern of preserved versus resampled grain into a pattern of lighter and dark bands like this.

The effect may also be exaggerated by an underlying checkerboard noise artifact in the sensor (that's been there from day 1 or has developed over time) or it may be something in the demosaicer.

Does the artifact show up in single, unprocessed, uncorrected RAW images? What if you develop the RAW image in a different piece of software?
aha, i did use the auto lens correction and that pattern does resemble the perimeter correction i've seen for that lens. I will go back and look at it without, and presume it will be a typical grid.

---------- Post added 02-11-2018 at 08:51 AM ----------

QuoteOriginally posted by BigMackCam Quote
Is it always in the dead centre of the image, Mike? Any filters in front of the lens? Happening with just one, or more than one lens?
no filters. always centered like that. i havent compared to other lens but will look.
02-11-2018, 09:53 AM - 1 Like   #6
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What software? There were something like this a year or more ago where the graduations in color were not handled property by software which didn't have enough color depth to deal with it. 16 bit vs 32 bit iirc.
02-11-2018, 10:09 AM   #7
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Yes, maybe you can try a software with a different demosaicing and NR algorithms. Stack the photos, and then apply distortion correction at the end (or not at all?)
02-11-2018, 11:13 AM - 1 Like   #8
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Thoughts, not answers:

It looks similar but not the same as "Newton's Rings". Interference pattern in your optical path? Did you use any filters?

Those 2 samples are with different lenses, 35mm and 55mm, correct?

Try taking dark frames and see if you can get a similar pattern. That might help determine whether the artifacts are optical, sensor-based, or from software processing.

02-11-2018, 02:54 PM - 2 Likes   #9
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I have had this problem with stacked star trail images. It went away when I turned off ACR's lens corrections when doing the RAW conversions. Suggest giving that a try!
02-12-2018, 09:00 AM   #10
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I was out of pocket all day yesterday but thanks for so many comments on this item.

I suspect as photoptimist suggest, this is the sensor grid pattern that appears when gained up, and the strange bullseye shape is result of the distortion lens correction in ACR, which is why it looks so unusual. I doubt there is any fix for this when pushing exposure so much in extremely low light situations.

Thanks again all!
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