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12-15-2009, 09:53 AM   #1
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Long exposures dangers in DSLRs: Myth or truth?

Hello,

I have done up to 2 minutes exposures with my K100DS without any problems but I'd like to do super long exposures for star trails. Something in the range of an hour or maybe even more.
Is it true that it is not advisable to do super long exposures in DSLRs because the sensor can overheat and get damaged? Any truth to that?
Any other concerns? I think battery life is a consideration.

Thanks,

12-15-2009, 12:48 PM   #2
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Short answer, no.
No damage done AFAIK. And I have done many long exposures, up to 1 hour.
I don't have a power adapter for either of my cameras, which stops me from going for longer exposures, but I would if I did.
12-15-2009, 02:00 PM   #3
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AFAIK, the only drawback to super-long exposures is that the camera does a "dark frame subtraction", to reduce noise. This means that, after the exposure, it closes the shutter and takes another dark frame of the same duration. Then it uses that dark frame to identify and remove the noise from actual image.

This means that a one hour exposure takes two hours to make.
12-15-2009, 02:13 PM   #4
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There is no danger whatsoever. Otherwise, P&S cameras would explose everyday...

12-16-2009, 08:26 AM   #5
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The DFS can be turned off on the K100ds.
12-16-2009, 08:47 AM   #6
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No danger BUT.
k100d and others using the 6Mpix sony CCD sensor, have a very pronounced dark current - that is some "zero" current that simply leaks trought the photisite.

This current produces heat, but you may know that heat increases dark current - producing even more heat, so theoretically there is no limit for this cycle, unless at some point more heat is dissipated away from the sensor, than is being produced.

Still if there is a point where dark current could not further "regenerate" and increase to undefined level, you'd get a hell lot of noise in longer exposures with k100d anyway.

Here i found some interesting graphs:
K100D is way noisier than the K200D on Flickr - Photo Sharing!
It seems, that even if k200d has much better performance it's temperature is still ramping up with exposure time.

Fortunately, Pentax CCD cameras does not have the limitations of newer models, where the dark frame subtraction is mandatory at some point. If you have turned it of, then it will never happen, even after an hour of exposure.
12-16-2009, 08:56 AM   #7
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QuoteOriginally posted by noblepa Quote
AFAIK, the only drawback to super-long exposures is that the camera does a "dark frame subtraction", to reduce noise. This means that, after the exposure, it closes the shutter and takes another dark frame of the same duration. Then it uses that dark frame to identify and remove the noise from actual image.

This means that a one hour exposure takes two hours to make.
AFAIK you can turn that feature off on all cameras but the K-7.
12-16-2009, 09:10 AM   #8
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QuoteOriginally posted by ismaelg Quote
but I'd like to do super long exposures for star trails. Something in the range of an hour or maybe even more.
There are other ways to do star trails too, but it depends on how you want the sky to look (e.g. black or blue). For the photo below I just plugged in my remote trigger to my K10D and set it to keep firing 30-second exposures. I came back about 45 minutes later and had a bunch of exposures. I combined them in separate layers in PS using Lighten Mode.


12-16-2009, 09:56 AM   #9
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Hi Derek,

Not to hijack the post..... but I really like to know how to set the camera to take this kind of shot?
what is the f stop and how did you focus? I have tried several time to do this kind of shot but end up with dark and blurry images --- and hours of putting my self in freaking cold weather...LOL

Thanks,
Lee
12-16-2009, 12:05 PM   #10
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Hello Lee,

I turned off noise-reduction for long exposures in the menu of my K10D, then set my camera on a stable, heavy-duty tripod late at night (about 1:30 a.m.) away from the city to avoid light pollution. I aimed the camera at the North Star (because that's near the northern axis of the earth) to get the circular motion look and set my camera to manual focus.

I turned the focus ring on my FA 43 to infinity, then slightly back the other way (around one degree-but this distance will vary with each lens) and set the aperture to f/2. I set my camera's ISO to 200 and used my home-made remote (Intructions here) to keep the shutter firing every 30 seconds (30s exposures). It was really cold outside, so I went in where it was warm and came back outside 45 minutes later to switch the remote off.

Then as I mentioned earlier I combined the photos as layers in PS using Lighten Mode, merged and saved as JPG. If you have any questions please let me know.
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