This is the second half of a reply I made last week; for some reason, the Quote feature won't work with the entire post, but does broken in into two halves. Apparently there is a limit on the number of Quote delimiters in the software.
The relative contributions of photon noise to read noise at unity gain vary with signal strength. With higher signals, the noise source is mainly photon noise, while read noise predominates at lower signal strength. You did not state what signal strength was involved in your statement about read noise at unity gain.
Perhaps that's because it is irrelevant. Shot noise is not a wild card; shot noise is dependent only on the geometry of photon collection and quantum efficiency, including the effects of filters. Read noise is the thing that varies greatly from one camera to another, and its relationship to ISO varies greatly on any given camera. None of the things that you do to improve read noise have any effect on shot noise. The less read noise you have, the more usable your shadows become.
Of course, with a dark frame with a short integration time, the noise is almost entirely read noise, since there would be no photon noise and thermal noise would be minimal.
You've just described my blackframes! Why all the confusion at the beginning of your reply?
To use Roger's data for the 1DM2, full well at ISO 50 is 80,000 electrons. At ISO 1600 (unity gain), full scale on the A/D converter would represent 2500 electrons.
No; that's not Roger's data. Roger is well aware (pun intended) that RAW saturation at ISO 100 is not half as many photons as well
saturation at ISO 50. ISO 50 is really only capable of about ISO 70 to 75 on the 1Dmk2, but is exposed like it is actually ISO 50, leaving less highlight headroom than the other ISOs, relative to metering (a major engineering blunder, IMO). RAW saturation at ISO 1600 on the 1Dmk2 is approximately 52300 (by his data) divided by 16 = 3269 electrons. I don't know if his values are correct, though, because he may assume that the 1Dmk2 has 4095 RAW levels to use, but signal is only represented by ADU 128 through about 3700+. He seems to derive either ADU:electron "gain" or max photon count from the other, using the uncertain figure.
The variance of photon noise would be 2500 and the variance due to read noise would be 15.2. If you look at noise in the shadows, for example 6 EV down, one would collect about 40 electrons. The variances of the photon noise and read noise would be 40 and 15.2 respectively. Photon noise still predominates.
No. Six stops down from saturation would be 3269/(2^6) = 51.1 electrons, which would have a shot noise of 7.15 electrons. Total read noise is about 4.19 electrons. Read plus shot would be (51.1 + (4.19)^2)^0.5 = 8.29 electrons, or about 0.21 stops more noise. How far down is Sat-6EV, though? There is about 3.5 stops of headroom above metered grey in most Canon DSLRs (4.0 for the XTi). That puts Sat-6EV about 2.5 stops below middle grey; not very far into the shadows at all, and that's just the green channel. A chain is no stronger than its weakest link, and the red channel is 1 stop less sensitive in daylight WB, so Sat-6EV is only 1.5 stops below "middle red". Change the WB to tungsten, and the blue channel is only 0.5 stops below "middle blue", with Sat-6EV!
Now, let's talk about some *real* (but still not extreme) shadows: 4 stops below middle blue in tungsten lighting - that's Sat-9.5EV. 3269/2^9.5 is 4.51 electrons. That's a shot noise of 2.2, against a read noise of 4.19. Total noise is (4.51+(4.19)^2) = 4.7, or 1.09 stops more total noise.
Not to mention the fact that the read noise has highly-perceptible line noises not present in shot noise of equal intensity.
Furthermore, when one determines read noise, one is measuring total noise and the various contributing components can not be separated out, and your statement "The noise introduced by the ADC unit, alone, would be cut in half, relative to signal, if there were a true gain-based ISO 3200" does not make much sense since you can't separate out amplifier noise from ADC noise.
It is not clear how much of the noise is due to the ADC, but ADC noise is real, and whatever part it is, would be reduced 50% relative to signal, a small, but real improvement. Total noise at ISO 3200 would be (non-ADC-1600-noise^2+(ADC-1600-noise/2)^2)^0.5. You don't even need any more at-sensor gain to get this small benefit, so the issue of the point of diminishing returns of sensor-read amplification is another issue. Some manufacturers use amplification of some kind for ISO 3200; the Minolta K7 a case in point; it's ISO 3200 has less noise than 1600 under-exposed by a stop.
Finally, the proof of the pudding is in the eating, and you should measure noise at various ISOs rather than theorizing at what they might be.
I did; what are you talking about?
Astronomers who have done their homework apparently do not bother with amplification much above unity gain, since it has no advantage with respect to noise and merely reduces dynamic range.
That's a very vague statement, and it may apply to old technology, and/or equipment that doesn't have any gain above unity gain, because the manufacturers believed the same myth, or technology wasn't ready.
The bottom line, which you seem to ignore, is that the steps of the ADU are arbitrary relative to the analog equivalent of an electron.
The unity gain of the Canon cameras is relatively high, and the differences might be demonstrated more easily with a camera with a lower unity gain (such as the Nikon D200 with a unity gain of 800).
Go ahead, demonstrate it, and I will offer alternate explanations for your results.