Let's have a look at Bill Janes rebuttal of some points in Michael's essay, in reply #114, to see where there might be a flaw in Bill's logic, or an incorrect statement.
In particular, the following comment from Bill looks a bit suspicious:
However, he (Michael) never quite comes around to the fact that the number of discrete and distinguishable levels is limited by noise throughout the image from highlights to shadows (disregaring read noise) rather than the number of levels in the raw file.
What does Bill mean by distinguishable? I presume he means
visually distinguishable otherwise there would be no need to write 'discrete and distinguishable'
Now this statement from Bill is contrary to my understanding of the situation, which is that the number of levels the camera can record in the brighter stops, despite the presence of noise, is far greater than the eye can distinguish. However, the number of levels the camera can record in the darker stops is far fewer than the capacity of the eye to detect, even without the presence of noise.
Those who insist that it's only the reduction of noise that counts, not the increase in available levels, should consider what would happen in the situation of a theoretical noise-free camera. Let's imagine that all internal camera noise has been eliminated with sophisticated noise-cancellation techniques, and let's imagine that Emil Martinec has invented a device that fits over the lens to remove all photonic shot noise.
A principle of a 12 bit DSLR is that the brightest stop can contain 2048 recorded levels and the darkest stop just 1 level, giving us a theoretical 12 stop DR.
Does anyone really believe that the human eye can distinguish no more than one level in the deepest shadows of a 12 DR scene?
As I understand, the human eye has a tremendous capacity to adjust to changing lighting conditions. When the gaze shifts from the brightest part of a scene, of 12 stops DR that we are about to photograph, to the darkest part of that scene, the pupil dilates, if not instantaneously then within a few seconds, and we see more detail in that darkest shadow, if the detail exists. Certainly far more than the 1 or 2 or 4 levels that a 12 bit DSLR is able to record in those 3 darkest stops, irrespective of noise considerations.
If we want the camera to record say 16 levels in that darkest stop, which the eye has seen in the real scene and which we want to capture, and which may in fact turn out to be only 8 or 10 levels of real information, the rest being noise, we have to overexpose by 4 stops and sacrifice the highlights, or bracket exposure for merging to HDR.
I maintain that ETTR is all about both noise
and the number of levels.