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A dark frame is usually taken by placing the lens cap on the camera and opening the shutter for a period of time to determine the noise component of the signal for a certain exposure duration.

I know the camera software is written such that no photos can be taken with the shutter closed, but is it possible, in theory, to activate the sensor with the shutter closed and use the result as the dark frame?
Admittedly this would require a script or software package to be installed on the camera, but is this a viable method for taking a dark frame?

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See these search results: Search string "long exposure noise reduction" –  Esa Paulasto Mar 24 at 19:55
    
Is it important that you don't activate the shutter? Are you trying to take dark frames just for the sake of it, or are you trying to get dark frames manually between light frames on a regular basis? (one trick is to just put the body/lens cap + viewfinder cover on and take the shot like that) –  drfrogsplat Mar 25 at 0:29
    
Why does it matter if the shutter is opened? If the lens cap is in place, it should be the same really. I guess I'm missing the value of your desired approach. –  John Cavan Mar 25 at 3:04
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Because the dark frames will be done in the middle of a time-lapse series where the lens cannot be put on the camera due to possible interference with non dark frame exposures. Plus that would mean standing right beside the camera to continuously put the lens cap on and off over the period of 4 to 8 hours. –  Jon Mar 25 at 3:16

2 Answers 2

This is not only possible, but is in fact the way that some noise cancellation modes already work. Long exposure noise reduction (or LENR) will automatically take a dark frame of the same exposure length after a long exposure image capture. It then uses this dark frame for noise cancellation.

More detail on LENR is covered in this question.

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Yes, LENR is a feature so common that even the cheapest entry level Sony SLT has it, and I have it turned On for every night time long exposure photo I take :) Then again, to actually take a dark frame, so that you have the dark frame "image" file on your disk, that is not what LENR does. –  Esa Paulasto Mar 24 at 19:27
    
@EsaPaulasto - thanks, updated answer to reflect it as a general term. That's true that it doesn't save it out, but really, the dark frame should be from as close to the image being taken as possible for best result. I suppose it would probably be possible for a third party firmware to save out the LENR image before it is discarded after the subtraction. –  AJ Henderson Mar 24 at 19:41
    
@EsaPaulasto - hmmm... done right the exposure time matches and I've taken hour plus long exposures before. That's a painful experience to say the least. –  John Cavan Mar 25 at 3:02
    
@JohnCavan Yeah, I can imagine. My longest exposures so far have been only some minutes, and even then the waiting for LENR to finish feels like it's taking forever. –  Esa Paulasto Mar 25 at 4:08

Yes, that should work fine, and is most likely something the processor could do, just that this capability has not be surfaced to the user interface of most cameras.

Even if so, it would still be a good idea to put the cap on the lens to prevent any light getting near the shutter in the first place so that it can't somehow leak around to the sensor. It would also prevent accidental burning of the shutter if the lens got pointed at the sun.

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I did not even think of the built in long exposure noise reduction as an example to show it is possible. However, this will unfortunately not suffice for my application, since I am considering it for time lapse and I cannot have a dark frame for each photo as it would leave a big gap between frames that makes the movie choppy and it also halves the number of exposure that I can take in a given time frame. I may have to consider write software to interface with the camera to allow it to take a dark frame ever user specified number of images and subtract that data before writting to raw files. –  Jon Mar 24 at 19:19
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@Jon - what are you time-lapsing that you can't adjust the exposure settings to allow for the needed time? The idea with time lapse sequences is generally that you have a gap in time between each frame. It won't make it jumpy as long as the interval is still regular. The usefulness of a dark frame is going to go down significantly as the time between the dark frame and the image itself grows longer. –  AJ Henderson Mar 24 at 19:24
    
Intervals between end and star need to be relatively small for night time time-lapse or you have to speed up the frame rate since the stars will jump a noticeable amount between frames, although the time will vary for different lenses. The issue is not exposure, in this case unless I am changing lighting conditions I need to keep the exposure constant in order to collect the right amount of photons. –  Jon Mar 24 at 19:52
    
The uselfullness does go down, but at the very least it will carry some representation of the thermal noise as well as the read noise which should not change from frame to frame. The question that I would like to answer is "Does using a dark frame from 20 or so frames ago create a better quality image than without noise reduction, just not as good as it would have been with a recent dark frame, or does it make a worse image than with no noise reduction?" I am considering experimenting with this. –  Jon Mar 24 at 19:56
    
@Jon - this didn't occur to me earlier, but your n'th photo idea for a dark shot would actually be much worse. It would produce an uneven interval and would make the finished video feel like it dropped every n'th frame. –  AJ Henderson Mar 24 at 20:07

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