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06-06-2010, 04:30 PM   #1
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[langtitle=zh-CN]A long time exposure strange phenomenon[/langtitle]

First say hello to everybody, and I am Very glad to join this big family.

When My K7 in "B" mode, waiting for a long time after exposure till the camera could review the photo, at this period, the camera has a bump bumity noise.

The voice is not big, but if the ear near the camera, the sound is very clear.

But there is another very strange phenomenon, I feel this sound is not like an electric current signals, it much like mechanical vibration voice, and the SR function are switched off, so more strange.

I hear that some users mentioned K7's long exposure NR function, but I disabled it, why there still met this phenomenon?

In addition, even it comes from the NR function, I guess it is the signal processing, why would feel is mechanical vibration voice?

Are other friends have similar situation?

Waiting the experts to answer the questions!


06-06-2010, 05:51 PM   #2
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It seems to be normal, my camera does the same thing. I don't know what is making the noise.
06-06-2010, 05:57 PM   #3
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I believe that what you are experiencing is the "dark frame subtraction". After long time exposures, the K-7 takes a second exposure, but with the shutter closed. The duration of this exposure is the same as the original exposure.

The process is that, since the shutter is closed, any values in any pixels of the sensor on the second exposure, are noise. This noise value is "subtracted" from the first image, to, hopefully, arrive at a less noisy image.

On the K10D, this can be turned off, but I've heard complaints that on later cameras, specifically the K-7 and K-x, that it can not be turned off.

I don't know why it would make any noise.
06-06-2010, 10:53 PM   #4
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Just the other day I read on PF that even if the shake reduction is turned off it has to be on to keep the sensor still. Otherwise it would be all over the place in the camera aside from on a tripod which it would then be in the bottom of the camera. The shake reduction makes some mechanical noise.
Could that be it?

06-07-2010, 03:29 AM   #5
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QuoteOriginally posted by StarDust Quote
Just the other day I read on PF that even if the shake reduction is turned off it has to be on to keep the sensor still. Otherwise it would be all over the place in the camera aside from on a tripod which it would then be in the bottom of the camera. The shake reduction makes some mechanical noise.
Could that be it?
That sounds like a logical explanation to me--the sound of the sensor being kept in place as opposed to the sound of the sensor moving to compensate for shake.
06-07-2010, 09:43 AM   #6
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QuoteOriginally posted by noblepa Quote
I believe that what you are experiencing is the "dark frame subtraction". After long time exposures, the K-7 takes a second exposure, but with the shutter closed. The duration of this exposure is the same as the original exposure.

The process is that, since the shutter is closed, any values in any pixels of the sensor on the second exposure, are noise. This noise value is "subtracted" from the first image, to, hopefully, arrive at a less noisy image.

On the K10D, this can be turned off, but I've heard complaints that on later cameras, specifically the K-7 and K-x, that it can not be turned off.

I don't know why it would make any noise.
Forced auto-DFS is present in the K20D and any newer Pentax camera.

This means that regardless of the menu settings, auto-DFS is performed when:
1) You take a bulb exposure longer than 30 sec
2) You take a normal exposure longer than 2-3 sec

*#@$$)#@*$@ annoying since only one dark frame reference is needed to support multiple shots. I still go back to my K10D for fireworks photography because of this.
06-07-2010, 07:43 PM   #7
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Thanks to these friends, and I now know the reason from your replies.
Could we report this issue to Pentax and ask them to do some change in the new firmware?
06-08-2010, 09:18 AM   #8
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You could, but many have, and it's been a couple of years now since the K20D came out. The assumption is that noise would be so bad Pentax just won't allow it on those cameras.

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