I have spent a good part of a slow day reviewing some relevant papers on the topic of what we can actually see in terms of CR as it relates to wellwatching a movie. Most of the links mentioned kinda miss the mark so I thought it better to return to the fundamentals rather then the details of human eye physiology which lose me anyways.
I downloaded these papers and unfortunately they can't be posted because I had to buy them:
1. Local luminance and contrast in natural images
Robert A. Frazor 1, Wilson S. Geisler; Vision Research 46 (2006) 1585-1598
2. The statistics of natural images
Daniel L Rudermant; Network: Computation in Neural Systems 5 (1994) 517-548.
3. Contrast Gain in the Brain
; Geoffrey M. Boynton, The Salk Institute
Summing up the main points is a difficult task at best. I usually have to read papers such as these several times to get the most out of them but there are a few points I can share.
All of these papers discuss something called contrast normalization or contrast gain control. It is the process where the eye and brain adapt to the contrast range in a given scene by finding the mean level of contrast and it occurs very rapidly, about 20 ms or so. Contrast discrimination occurs around the mean and is done partly in the visual cortex.
There is a distinction between local luminance and local contrast in a natural scene and they do not correlate. I think this is why the researchers in one of armadillos links decided to make luminance a constant because it doesn't seem to matter.
It is the difference
in light levels not the light level itself that is important; this difference is contrast. Photoreceptors in the eye have a dynamic range of one to two orders of magnitude in detecting light intensity; however, do not mistake this statement to mean that the eye can only see a 100:1. That would imply that only the receptors in the eye are involved in contrast perception and it is clear to me that the brain has a big role as well and maybe what increases the perceived ratios to much higher than that, I am still unclear on this.
The Contrast Response Curve (not CSF) is the S shaped curve we are familiar with and shifts over a backdrop of a much larger contrast difference. By and large the curve stays the same as it adjusts to the mean. So the range doesn't actually increase per se it just moves around.
After reading these papers and others it is not obvious whether the detectable ratio is 130:1 or 1000:1 or somewhere in between. I'll try to tease it out of the math if possible but it is not trivial. The HVS is a very complicated thing and scientists freely admit that there is much they do not know.
CORRECTED: 2 ms to 20 ms