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08-04-2011, 09:31 AM   #1
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Another 645D Question

I've looked repeatedly through the 645d manual but I cannot find an answer to this puzzle. The other evening I was doing some night sky shots with long exposures, Bulb, with an intervalometer.

After completing exposures ranging from a minute to two minutes, the camera went completely dark as if turned off. At first I thought the battery had died, then looking at the top LCD display with a flashlight I discovered a numeric countdown taking place. Upon reaching 0, the camera came alive again.

Is this possibly a cool down period for the sensor before operation is allowed once again?

08-04-2011, 11:39 AM   #2
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It is DARK FRAME SUBTRACTION aka Slow Shutter Speed NR. It is normal. On exposures longer than some time (probably varies with camera -- on my K20D it's 0.3 second) the camera takes another exposure of the same duration, but with the shutter closed. Then the dark frame is subtracted from your shot. The purpose is to eliminate noise caused by sensor heat.

In short: Don't sweat it. Just remember that any long exposure takes TWICE as long!
08-04-2011, 11:45 AM   #3
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I think you can turn this off.
08-04-2011, 04:40 PM   #4
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I tried to turn it off in the menu as I did my first bulb shots this evening but it did it anyway...The menu says it's switched off. Was using a kenko ND400 with it...

08-04-2011, 07:42 PM   #5
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Thanks! I did a search a few minutes ago and came to the same conclusion.
08-05-2011, 02:30 PM   #6
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QuoteOriginally posted by itshimitis Quote
I tried to turn it off in the menu as I did my first bulb shots this evening but it did it anyway...The menu says it's switched off. Was using a kenko ND400 with it...
Strange, I only have K-5 and just looked at it. I have it turned off and after 40 seconds it is just ready in a few seconds (some lens corrections), but when I turn it on it doubles the time to proces.

It should be the same with 645D. In the menu page 3, the 4th row noise reduction long shuttertime.
08-06-2011, 04:25 AM   #7
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QuoteOriginally posted by RonHendriks1966 Quote
Strange, I only have K-5 and just looked at it. I have it turned off and after 40 seconds it is just ready in a few seconds (some lens corrections), but when I turn it on it doubles the time to proces.

It should be the same with 645D. In the menu page 3, the 4th row noise reduction long shuttertime.
I've set that to off and it makes no difference. I've emailed the Operations manager at Pentax UK about it. He was very helpful when my camera was faulty
08-06-2011, 10:02 AM   #8
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QuoteOriginally posted by RonHendriks1966 Quote
Strange, I only have K-5 and just looked at it. I have it turned off and after 40 seconds it is just ready in a few seconds (some lens corrections), but when I turn it on it doubles the time to proces.

It should be the same with 645D. In the menu page 3, the 4th row noise reduction long shuttertime.
QuoteOriginally posted by itshimitis Quote
I've set that to off and it makes no difference. I've emailed the Operations manager at Pentax UK about it. He was very helpful when my camera was faulty
Maybe the 645D is like the K-7 in that regard. You can turn it off but anything longer than 30 seconds and it does it anyway?

08-12-2011, 09:01 AM   #9
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i run into exactly the same problem on 645d. never the less everything related to NR is TURNED OFF camera ignoring this and applying noise reduction... weird.. Pentax keeps silent..
08-12-2011, 12:27 PM   #10
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Official Pentax answer regarding 645d:

QuoteQuote:
On the 645D, with its CCD sensor, you cannot turn off Noise Reduction in long exposure shooting.
QuoteQuote:
Here's a more detailed description of how the NR works in the 645D from our corporate office:

For those exposures where long exposure NR is activated (i.e. exposures longer than 30 seconds), the camera will display a countdown timer on the top LCD screen showing the remaining NR processing time. For example, when the shutter closes after a 2 minute long exposure, the NR processing will begin and the LCD will display 120 seconds and start counting down. When the countdown reads “0” the NR processing is done and the image will be written to the memory card.

Long exposure NR involves taking an additional image (a dark frame subtraction image) that the camera then compares to the original image and uses to reduce any random pixelization caused by the long exposure. The total time for images using long exposure NR is double the time for a single image. A 1 minute exposure will involve a second 1 minute NR exposure so the total time is 2 minutes. A 1 hour initial exposure will mean a total processing time (including NR) of two hours. This often affects users doing astrophotography as the exposure times can be very long, but there is no way to circumvent NR processing for exposures of 30 seconds or more.
08-12-2011, 01:04 PM   #11
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Pentax Uk told me to turn it off in the menu. Pretty daft as this doesn't work. I haven't as yet upgraded the firmware but from reading here that doesn't solve the issue.
08-12-2011, 02:25 PM   #12
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QuoteOriginally posted by itshimitis Quote
Pentax Uk told me to turn it off in the menu. Pretty daft as this doesn't work. I haven't as yet upgraded the firmware but from reading here that doesn't solve the issue.
aha... i have the latest firmware v1.11, that does not help..
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