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01-21-2009, 07:54 AM   #1
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Strange K20d High ISO noise patterns

Hi.
I've recently upgaded from a K100d to a K20d and am enjoying it very much. However, I've noticed a strange pattern of noise appearing in all my pictures taken at high ISO (3200 and up). It's awful by the time I've reached 6400.



I know I shouldn't expect wonders at ISO that high, but I would have expected the noise to be evenly spread across the image. Instead, I get a 'purple glow' along the bottom egde and top left corner of pictures. It looks like long-exposure amp noise, but appears on fairly short exposures.

A page of boring example shots from ISO 100 to 6400 can be found here:
K20d ISO noise

Has anyone else experienced this? I have the most up-to-date firmware. I don't use these ISOs much,and otherwise the camera's great, but I will use them occasionally and this makes the images unusable.

Many thanks,
Mike

01-21-2009, 08:13 AM   #2
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My k20d has the same kind of noise in the high sensitivity images.. A blue glow on each long side.
01-21-2009, 08:18 AM   #3
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Same thing here. You can often see it clearly on darker areas even at iso3200. I Should test if it appears past iso1600.

I was wondering if this is normal on all k20ds or could this be a warranty issue?

pdap
01-21-2009, 08:34 AM   #4
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Here's what I get (100% shot of the fridge at ISO 6400, raw, no NR, histogram peaks left of centre, converted in Raw Therapee without any correction)


The shadows are because the scene wasn't illuminated evenly.

If I look really hard I see a tiny bit of horizontal banding, but it's pretty uniform and nothing like yours.

01-21-2009, 09:05 AM   #5
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What is even more interesting (to me) is that the K20D has a strange vertical bar at the left side of the image, visible (normally) only by extremely pushing the contrast and brightness.

So far I was able to produce it with every K20D I saw.

01-21-2009, 10:02 AM   #6
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try isolating the green channel if anyone has photoshop, the green channel has patchy noise whereas the others have harsher grain


Also, to anyone reading the thread, properly exposing photographs does wonders for noise. A bright image properly exposed at 3200 can be perfectly useable.
01-21-2009, 12:50 PM   #7
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Isaac is right. Underexposure pops the chroma noise like crazy at the higher ISOs in the K20D. But properly exposed, you will only notice the grain in the shadow areas of the frame.

Jack
01-21-2009, 01:49 PM   #8
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its funny but the image looks a lot like this background

are you sure this isn't a desktop

VirtualVillage.com

01-22-2009, 02:24 AM   #9
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Thanks for all your replies.

Isaac: the noise pattern is in the blue channel.

I've added the three channels to this page:
K20d ISO Noise

Lowell: the image is not a background. I took all the pictures yesterday. It's actually of the side of a workbench in my lab (I'm a Physics teacher). I chose it becuse the noise is easiest to see over a dark background.

Jack: it is present in light pictures too. I took similar pictures of my whiteboard and the purple glow is still there, but as it appears as a lightening of the blue channel, it was less noticable as the blue channel was already light.


I think I'll contact Pentax to see if it is a warranty issue. I've only had the camera for two weeks.

I should have said that all these pictures were shot in manual mode with the DNG setting and imported into Lightroom. There has been no editing other than ensuring the white balance was consistent for all the shots (taken under fluorescent lights with auto white balance) and resizing for web. No changes were made to the exposure in lightroom. The nose was clearly visible on the camera's screen which prompted me to take these test shots.

Thanks,
Mike

Last edited by marnold; 01-22-2009 at 02:31 AM. Reason: Addition
01-22-2009, 02:28 AM   #10
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Here's what mine looks like when shot in dark room

01-22-2009, 02:51 AM   #11
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Just in case:
The left vertical bar can be seen best at ISO 100.
Pointing into the dark.
And you have to compress it quite a bit.

Quite academic, but interesting.

01-22-2009, 11:12 AM   #12
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Here is a shot even at iso 2000. No other corrections but the white balance and resize. Blue area is about one fifth or fourth of the frame in portrait position.
Without setting the white balance the picture looked quite ok. Just some blue visible on the hair, but when corrected the blue noise starts quickly showing up. Other channels look quite clear. At iso 3200 it can be a lot more noticable too. For example on dark areas or red textures.



bdip
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