Nikon D810 long exposure noise reduction

There has been some discussion on the web about long-exposure noise reduction in the Nikon D810 that occurs even when the menu setting for such process is set to off.  I have seen some indications of this kind of processing in my dark-field versus shutter speed tests.

Now that I have a tool for analyzing the spatial spectra of images, I can take a look at some of the Nikon dark-field images made when I took the data for these graphs.

First, the dark-field spectrum for the green channel, 1/8000 second at ISO 1000:

Except at very low spatial frequencies, the spectra are flat, which indicates white noise, which means no spatial filtering.

Here’s the histogram of that image:

There’s a little clipping at a bit over 570, and there are dropouts in the red and blue channels because of Nikon’s 14-bit digital white balance prescaling, but the histograms look normal otherwise.

Now at 15 seconds with the camera set for no long exposure noise reduction:

There is essentially no low-pass spatial filtering taking place.

The histogram of the 15 second, no NR image:

A little different, particularly the double-high bucket at around 600 in the red and blue channels. You’d expect this with digital prescaling, and I don’t know why it didnt’ occur with the 1/8000 second image.

Now with long exposure noise reduction invoked:

Quite a bit of spatial filtering.

The histogram:

There’s only one empty bucket. Thus, the spatial filtering is not median filtering, which cannot fill holes in the histogram; it must be some kind of averaging filter.

I’ll do some more testing and report.