Images were taken in lossless compressed 14-bit mode. Not that it is probably pointless to use 14-bit and lossless format for higher ISO value, given that the noise overwhelms the precision. However, ISO 50 and 100 and perhaps ISO 200 can likely benefit from 14-bit lossless files. ISO 50 in particular seems exceptionally “clean”.
The sample crop chosen was not necessarily optimally sharp as it was well behind the focus point, but it is reasonably sharp for our purposes of evaluating loss of detail with higher ISO.
In-camera long exposure noise reduction was used (this applies beyond exposures of a few seconds); the D3x uses a form of dark-frame subtraction as do all the DSLRs.
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