I’m wrapping up my work on spatial frequency analysis of read noise, and in this post, I’ll summarize what I’ve found and provide links to posts with the details.
In all the cameras that I tested:
The spatial spectrum of the noise when the camera is exposed to a dark field is not white, but has higher energy at lower spatial frequencies than white noise. In this regard, the dark-field noise is different from photon noise (aka shot noise, aka Poisson noise) and photoreceptor non-uniformity (PRNU).
In this post, and in many of the ones that preceded it, I refer to dark-field noise and read noise interchangeably. Though I wish it were the case, this is not strictly accurate. In some cameras, the black point is subtracted from the real raw readings before the “raw” file is written. This process removes half the read noise, chopping all counts below the black point off and assigning them the value of the black point. As it turns out, aside from making the dark field images look twice as good as they really should, this practice has remarkably little effect on the spatial frequency characteristics of dark-field images.
The low-frequency component of the read noise is more visually objectionable than white noise. In fact, with most cameras it’s downright ugly. If you click on the links to each camera above, the next page shows images of that camera’s read noise after low-pass filters of various shapes and sizes.
In the cameras I tested, the low frequency content of the read noise is worse from top to bottom than side to side, or vice-versa. This seems to depend on the design of the sensor, and gives rise to the image defect that photographers call banding.
Fortunately, in the cameras that I tested, the level of the low-frequency part of the read noise is far enough down that you don’t see it in normal images. In the case of the Nikon D810, it’s hard to see it even if you go looking for it.
In the Nikon D810, almost all the low-frequency read noise is the same from frame to frame, and can be subtracted out in post processing. In the case of the Sony alpha 7S, that is unfortunately not true.
Although read noise does change somewhat with shutter speeds, at least in the case of the Nikon D810, it hardly changes at all in the hand-holding range of 1/3- second to 1/8000 second. Even throughout the range of 1/30 second to 30 seconds, the amount of read noise doesn’t change much, and the frequency characteristics change hardly at all in the D810.