When I use DCRAW to extract the RAW pixel values from a NEF file made by my D5300 I notice that there seems to be a minimum value clamp = 588 applied to the pixel values. I am an astrophotographer so generating master BIAS and DARK frames is part of my image calibration method. Both BIAS and DARK frames are images with the lens cap in place so that no light enters the camera. I believe that Nikon uses a Darkness value of 600 in their NEF files. The 588 minimum value clamp causes a huge number of pixel values in my individual DARK frames to be clamped at 588. I believe this limits my ability to generate useful master DARKs which are median or averaged versions of 10 or 20 DARK frames.
Is the 588 minimum value clamp imposed on the pixel values when the camera writes the NEF file or is the 588 clamp imposed by DCRAW due to NEF file standards? I would like to be able to remove this minimum value clamp or, at least, lower it to 0. Can you enlighten me?
Peter W
Nikon D5300 black level value
Nikon D5300 black level value seems to be 600
-- Alex Tutubalin @LibRaw LLC
I agree that the black level
I agree that the black level for a Nikon NEF file is 600. For a 14b Canon CR2 file this black level is 2048. When I look at a Canon ISO 1600 300 second exposure DARK frame (lens cap installed blacking all light) I notice that there are pixel values much lower than 2048. If you view the Canon data as a histogram you see a full bell curve shaped histogram centered at roughly 2048. The values extend downwards to 1800 or more.
For a similar Nikon NEF DARK frame the histogram is centered at 600 but the entire left hand side of the histogram is "missing". These values have all been clamped at a 588 minimum. I need access to the full histogram of values so that I can average multiple DARK frames to reduce the random noise and isolate the DARK current signal. I can do this quite effectively with a Canon camera. Doing this with a Nikon camera is less effective simply because of the huge number of values clamped at the 588 minimum.
Dear Sir:
Dear Sir:
Could you please upload the dark frame from your Nikon for analysis and send us the link to info@libraw.org ?
On a side note, it is not always sufficient to cover the lens, for dSLRs I cover the viewfinder too. Covering the whole camera may result in increased heat buildup. I also remove batteries and use an external power source.
--
Iliah Borg
Yes...I understand the
Yes...I understand the precautions for taking DARKs. I also add a computer fan blowing air on the rear of the camera to limit the heat buildup in the camera.
Dear Sir:
Dear Sir:
I checked the file, black is indeed at 588, clamping is evident.
--
Iliah Borg
Clarification please
Iliah,
Are you saying that the RAW data in the file is clamped at 588 and that there is no possibility of recovering the unclamped values?
Peter
Dear Sir:
Dear Sir:
Sorry for being unclear. Yes, the raw data in the NEF file is clamped at 588. There are no values below, and no way of recovering the missing values from the data - they are not there. As far as I know hacks to obtain true dark current and to disable star eater are available for D5100 and D7000, but you can reach out to nikon hacker community for more up-to-date info on the subject.
--
Iliah Borg
Thanks for clarifying
Iliah,
I did notice that Nikon hacker claims to have a hack for the D5100 and D7000. I also e-mailed Nikon to ask if an "astronomy firmware" update could be made available for the D5300. Nikon has not responded yet, which is a shame because Nikon Hacker gives me the impression that it is possible to do.
Thanks once again for clarifying...it's nice to know that someone is continuing to support the efforts begun by Dave Coffin.
Peter