D90 Does Noise Reduction on NEFs at Longer Exposures
Prepared 2008-11-01 (150/223874) by Bill Claff

I analyzed pairs of "lens cap" shots taken with no noise reduction turned on in the camera at a variety of exposures (ISO 200).
I had previously analyzed data for the D200, D300, and D3.
And like the D200, D300, and D3; the analysis proves that the D90 performs noise reduction at longer exposures.

First, here is a chart of noise versus exposure time (in stops).
(There is an extra data point at 1/800s and one missing at 8s)


Note the drop in noise at exposures of 1s or longer.

To confirm that this is a result of noise reduction I performed 2D Fast Fourier Transforms (2D FFTs) on the center 256x256 portion of the images.

Here are the results for 9 different exposures:
(I used an aggressive Levels adjustment to make the patterns more obvious.)


The exposure times were:

1/15	1/8	1/4
1/2	1	2
4	15	30

Note the white centers starting to form at exposures of 1s and longer.
This is proof of (inter pixel) noise reduction.