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There are several questions and a tag relating to what it is but I can't find much relating to how it is actually done in camera anywhere. The internet reveals many "how to" pages for doing the processing in software which seem to be broadly the same method ie.

-Take 1 or (ideally) more dark frames and use them as a subtraction or difference layer over the image or light frame .

This is mostly used by astronomers and is fine for dark areas (the space between stars) as the noise is reduced to black but obviously not so good for long exposures with lighter tones where black pixels will become evident.

To counter this two variations seem to be used

  1. Subtract the dark frame from the light frame then use a despeckle filter targeting the the dark pixels.
  2. Use the dark frame as a mask and apply some blurring to a duplicate light frame to interpolate values for these pixels.

Does the camera use something similar to the latter, perhaps using the dark frame as a mask then interpolating the data in a similar way to the way de-mosaicing works? Merely subtracting the dark frame doesn't seem to be what I see in the results I get as I see no black pixels on light areas of the image.

  • Possible duplicate of Why does my Canon 700D take so long processing when I take a long exposure? The top voted answer says, "The readings for each pixel in the dark frame will be subtracted from the reading for each pixel in the first frame before sending the raw data to your memory card." That's pretty much it at the camera level. At the software level, the developer of each software can use the information however they want. There is no "standard" way. – Michael C Dec 23 '17 at 11:38
  • @MichaelClark thanks for the comments. This may be an unanswerable question for the reasons chili555 mentions ie the intellectual property of the camera companies, but something more than simple subtraction is going on. – dmkonlinux Dec 23 '17 at 17:18
  • A lot of other things related to NR are going on whether one is using in camera dark frame subtraction or not. – Michael C Dec 23 '17 at 17:35
  • The frame subtraction is done to the single luminance values for each pixel well. It is done before demosaicing and the interpolation of color information. A lot of what you seem to think is the result of in camera DFS is really the result of demosaicing raw data. That happens whether the raw data being demosaiced is unaltered raw data from a single exposure or is raw data that has had a monochromatic luminance value for each pixel from the dark frame subtracted from the monochromatic luminance value for each pixel of the exposed frame first. – Michael C Dec 23 '17 at 17:44
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I am confident that the exact method used by each camera manufacturer is different and company-confidential. All manufacturers have a large vested interest in getting the maximum image quality using methods that are undetectable and therefore not reproducible by their competition.

In the case of Nikon, several sources describe a simple subtraction; for instance: https://improvephotography.com/48889/long-exposure-noise-reduction-use/

Dark frame subtraction occurs when that reference file, the dark frame, is used to subtract the hot pixels from the image file that includes both the signal and noise, which is the photo you intended to capture, plus the resulting noise.

And also: https://www.nikonians.org/reviews/understanding-long-exposure-noise-reduction

When you enable Long Exposure NR and an exposure is longer than one second (eight seconds on older Nikon DSLRs), the camera will take two pictures with approximately the same exposure time for each. The first picture is normal. The second picture is a black-frame subtraction exposure, which is exposed for about the same duration as the first picture, but with the shutter closed.

The camera examines the noise in the black-frame subtraction exposure and subtracts it from the first, normal image.

I am unaware of any in-camera long exposure noise reduction (LENR) that uses a more sophisticated process, although exact methodologies may be and probably are proprietary and confidential.

Because astrophotography sometimes requires stacking many images, dozens or even hundreds, perhaps, it is usually preferable to take all the exposures without LENR and take a few dark frames at the end. The dark frame(s) can then be subtracted in software during post-processing. As well, sophisticated processing, noise reduction, despeckling, etc., can then be applied and compared as desired.

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