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09-28-2011, 08:23 PM   #1
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sudden increase in long exposure noise

I'm out in the field right now and have noticed my k7 is producing a whole bunch of noisey hot spots on long exposures to a degree I haven't seen before. I have nr turned off until after 30sec, which is not unusual for me, but even at 15sec at iso 100 the hot spots are there. I've been doing a fair amount of long exposures in that range at night the last week and this is the first time the behaivor has revealed itself. The hot spots are the same colors in the same spots frame to frame, so clearly the sensor is angry about something. Exposures over 30sec with the nr active don't exhibit the same problem, which is to be expected, but it isn't a realistic option for what I'm trying to do given my need for successive photos with minimal intervals between them in the 15-30sec range... anyway, has anyone experienced this kind of sudden finnickiness from the k7/k20 sensor before? I'm hoping it's not permanent because as I said, this problem is brand new even if the sensors general random noise characteristics haven't been great (hot pixels happen, but not like this!).

09-28-2011, 09:03 PM   #2
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I have the K20 and earlier this year I was experiencing a great deal of noise at ISO 100, in the very late afternoon and evening shots. Essentially ambient low light imaging (5 to 30 second exposures). Pentax (CRIS) under the extended warranty swapped out the camera's guts - the electronics, main circuit board and sensor. That helped a great deal, however it is not what it was when I first purchased the unit. I took it back a few weeks later and they said that it meets the Pentax specs. Also, I only shoot RAW.

Well, I am currently still considering taking it back in - again now some 6 months later. Still getting more noise that what I feel I should be getting, especially since I have 6 months left on the extended warranty. The noise is most noticeable when bracketing 5 frames with the + ev frames - the smaller ev range I span when bracketing, the better the noise is. The noise is especially highlighted when I stitch and/or stack images. Also, I can use Picasa to review the images and it essentially displays how the stitcher will treat the frame - really any of the stitchers - autopano pro, panorama maker, pano tools, ice, photomatrix pro, etc.

I can also take say a 5 frame panorama, and at least one of the frames will be noisy while the others are just fine. Go figure. This I get very irritated with since it screws up the whole pano. If I check the image in the rear screen, it looks just fine....

Anyway - that is my tale of woe. Otherwise - its a wonderful camera - I like it a lot. If Pentax were to fix the noise issue, I would be happy and skip thinking about a K5 and perhaps wait for the K3 (or what ever it may be)....

I attached an example K20, using the 10-17 at 10mm, f 3.5, 5 shot bracket, 1/2 second to 30 seconds (+4 step ev bias). Actually this was at ISO 200 - I have to go change this back to 100 on the body. Stacked using MS ICE.

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Last edited by interested_observer; 09-28-2011 at 09:25 PM.
09-28-2011, 09:25 PM   #3
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QuoteOriginally posted by interested_observer Quote
I have the K20 and earlier this year I was experiencing a great deal of noise at ISO 100, in the very late afternoon and evening shots. Essentially ambient low light imaging (5 to 30 second exposures). Pentax (CRIS) under the extended warranty swapped out the camera's guts - the electronics, main circuit board and sensor. That helped a great deal, however it is not what it was when I first purchased the unit. I took it back a few weeks later and they said that it meets the Pentax specs. Also, I only shoot RAW.

Well, I am currently still considering taking it back in - again now some 6 months later. Still getting more noise that what I feel I should be getting, especially since I have 6 months left on the extended warranty. The noise is most noticeable when bracketing 5 frames with the + ev frames - the smaller ev range I span when bracketing, the better the noise is. The noise is especially highlighted when I stitch and/or stack images. Also, I can use Picasa to review the images and it essentially displays how the stitcher will treat the frame - really any of the stitchers - autopano pro, panorama maker, pano tools, ice, photomatrix pro, etc.

I can also take say a 5 frame panorama, and at least one of the frames will be noisy while the others are just fine. Go figure. This I get very irritated with since it screws up the whole pano. If I check the image in the rear screen, it looks just fine....

Anyway - that is my tale of woe. Otherwise - its a wonderful camera - I like it a lot. If Pentax were to fix the noise issue, I would be happy and skip thinking about a K5 and perhaps wait for the K3 (or what ever it may be)....

What you're getting there seems to be intense but fairly random noise. I get some of that of course but the real problem are clusters of bright/hot pixels in specific places in every shot. I'm still out freezing my toosh off outside but i'll try to post an example when I get home
09-28-2011, 10:22 PM   #4
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Well, that is flippin' annoying. The hot spots I was seeing seem to ONLY appear on my LCD screen. Load it up in the raw editor and they flash on the screen and then disappear as you zoom in... I'm guessing they are a product of the LCD's preview image compression. Blah! So I ended up taking multiple 20 minute long exposures instead of stacking a bunch of short exposures for star trails.

09-28-2011, 10:43 PM   #5
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Well, no not really. This is a stack of 5 images. Two of the images - had a full frame of somewhat uniform noise, however when stacked the software let the noise bleed thru down in the lower corner.

Removing the two images, the -2 and -4 ev step images from the 5 frame bracket, and restacking produces the following image. All 5 images were taken automatically, back to back across approximately 2 minutes from the first image to the last, each with a 2 second mirror up. The stacking software is indeed pushing the individual images, which essentially amplifies the noise that is in the frame.

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09-29-2011, 07:19 AM   #6
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QuoteOriginally posted by v5planet Quote
Well, that is flippin' annoying. The hot spots I was seeing seem to ONLY appear on my LCD screen. Load it up in the raw editor and they flash on the screen and then disappear as you zoom in... I'm guessing they are a product of the LCD's preview image compression.
More likely it is the raw editor that identifies the hot pixels and remove them, most raw editors do that.
09-29-2011, 08:00 AM   #7
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Yeah, RAW editors will automatically fudge those out. If you shoot JPEG they will remain and need to be post-processed out.

These are normal, but I believe these will eventually increase their numbers over time.
Generally not a bad thing until you get so many that it starts affecting the final image.
Continue shooting RAW and you'll probably never notice them, other than when viewing on the LCD.
09-29-2011, 08:35 AM   #8
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QuoteOriginally posted by amoringello Quote
Yeah, RAW editors will automatically fudge those out. If you shoot JPEG they will remain and need to be post-processed out.

These are normal, but I believe these will eventually increase their numbers over time.
Generally not a bad thing until you get so many that it starts affecting the final image.
Continue shooting RAW and you'll probably never notice them, other than when viewing on the LCD.
Interesting, I am surprised ACR does this automatically, as I thought that was the whole purpose of dark frame subtraction. I have noticed hot pixels that flash away in the past when opening in ACR for long exposure images, but last night there was just an unreasonable number of them. I suppose it is still possible my K-7's sensor is aging ungracefully, even if it hasn't affected the final products yet thanks to Adobe's digital magic.

09-29-2011, 08:44 AM   #9
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Yeah, I've never see much need for the dark frame subtraction, nor the long wait it requires, since RAW seems to take care of these pretty well without it.

I'm not sure what voodoo RAW processors are doing and why it is not optional...
Although I am sure there is some amount of noise where RAW processing stops doing its thing and DFS would still be a benefit, but it hasn't been a detraction for me yet.

I've seen the same behavior in other RAW processing tools. So it isn't just limited to ACR.
09-29-2011, 09:02 AM   #10
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QuoteOriginally posted by amoringello Quote
Yeah, I've never see much need for the dark frame subtraction, nor the long wait it requires, since RAW seems to take care of these pretty well without it.

I'm not sure what voodoo RAW processors are doing and why it is not optional...
Although I am sure there is some amount of noise where RAW processing stops doing its thing and DFS would still be a benefit, but it hasn't been a detraction for me yet.

I've seen the same behavior in other RAW processing tools. So it isn't just limited to ACR.
My understanding is the K-5 allows you to turn off DFS, and I suspect future Pentax dslrs will allow the same. I'm convinced the only reason you're not allowed to do it on the K-7 and earlier cameras is that Pentax is too embarrassed to show you the results the sensor produces under such extreme circumstances without correction.

Last night I was trying to do star trails here in Seattle, and although you can see quite a few within the city limits, I knew that a single long exposure would create an upper limit on how long I could keep the shutter open, as eventually the sky would brighten to the point that the star trails would become dim or invisible. Without stopping down past the point where the stars disappear for being too dim in the first place, I found the upper limit on this to be about 20 minutes:


Stars Over The Gas Works

I'd rather have at least 40 mins worth of exposures, so I had prepared myself for a long sequence of 30"-ers to be stacked in PS later. Obviously I got scared by the hot pixels and went the single exposure route, but I will have to try again the next time it's clear here. Perhaps the ideal situation would be to be able to take a series of ten minute exposures without DFS, and then take my own DF shot for manual correction, but the K-7 doesn't allow this. Obviously the 30" limit without DFS creates something of a limit on how bright you can expose your foreground objects, alas.

Perhaps next time if I can move fast enough I will take a long series of 30" exposures, and then rapidly switch to bulb mode for a 10 minute one at a smaller aperture to be layered in later...
09-30-2011, 01:00 AM   #11
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Does the K-7 menu option to map out hot pixels do anything for you?
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