Canon ISO is simply the amplifier configuration you are starting from. You could as well start from ISO 6400 in Canon menu, override CMOS[0] and ADTG gains to the values from ISO 200, an apply some other tweaks until you get the same highlight detail as with ISO 100.
The end result is ISO 100 and you should compare it with Canon ISO 100.
Personally, I think most people would read it as having been reduced from the Canon ISO. Stock standard, everyday ISO, set via Canon menu.
The ISO rating displayed is that in Canon menu, ie: the pulled ISO.
Rather then some convoluted gain configuration that reaches the same value.
However, your point still stands.
If you don't do this, you end up saying you get e.g. 0.5 stops more DR at ISO 6400, which is simply not true.
I believe they call this, an honest mistake.
SNR from some more signal levels would be more interesting (the highlights are already clean; most of the noise is in the shadows).
If I compare these 2 exposures.
ISO 200 - 1/60s - f/8.0 (lens unscrewed) - ADU 13519 - stdev 187.6
ISO 400 - 1/60s - f/8.0 (lens unscrewed) - ADU 7682 - stdev 92.7
We can see that the
pure SNR is,
13519/187.6 = 72.063
7682/92.7 = 82.869
To me, this shows that we are not entirely shot noise limited. The same
exposure resulted in different noise levels. Whether both exposures are considered clean or not is irrelevant. Clearly, even at strong signal levels, there is a measurable difference. And furthermore, since the 2 exposures were identical, the noise difference is entirely electronic induced. ie: Shot noise is not a contributor.
If we compare to the original example I described.
If I have my maths right, The noise difference between ML ISO 100 and Canon ISO 100 in the highlights.
log2(14169/13255) - log2(158.2/148.7) = 0.0068 EV
We can confirm that the exposures are very similar. At least, if my maths is correct!
14169/148.7 = 95.286
13255/158.2 = 83.786
Even though the noise levels are different.
If I had a full blown lab setup where I could control light output precisely, to capture the nonlinear behaviour near saturation, then life would be all good. In the real world, I'm trying to do the best I can, with what I have. Both physical equipment and mental aptitude.
If my results are far removed from useful, then I am more then happy to stop clicking the shutter count higher and higher.
So, the absolute amount of FPN is lower with the tweaks applied, but it doesn't decrease as much as the random noise => it becomes more noticeable.
Indeed. This is an important distinction. The FPN is not being induced by these gain tweaks. Simply, other sources of noise are being reduced sufficiently, to make this noise more apparent.
Using the 2 images on the previous page, we can conclude that the ADTG stage is a significant contributor to total noise. CMOS amplification was increased, ADTG amplification was decreased, total apparent noise also decreased. We can conclude that the noise is not (significantly) related to components further down the signal chain, because the signal level delivered to these components was very similar.
log2(14169/13255) = 0.096 EV
Here, the nonlinear behaviour in the highlights might effect the noise level in the highlights, but since the rest of the signal is linear, the signal
difference between the point I measured, and the shadows, should also be linear. In fact, if I compare a darker region of the 2 exposures.
log2(2722/2651) = 0.038 EV
This shows that as we move further towards the noise floor, the signal level between exposures becomes closer. Further emphasising that the ADTG stage is a significant contributor to total noise.
If we compare the noise in the shadows from 2 identical exposures, 1 @ ISO 100 and 1 @ ISO 1600, we know from plenty of past examples that the apparent noise level reduces.
Now, if we compare the noise from 2 different exposures, ML ISO 100 - 1/15s and ML ISO 1600 - 1/250s, where we haven't used ISO to increase the rendered brightness, but instead, have used ISO to brightness match 2 different exposures, the result is this.
https://dl.dropboxusercontent.com/u/34113196/ML/ADTG/Compare/ML1600.jpghttps://dl.dropboxusercontent.com/u/34113196/ML/ADTG/Compare/ML.jpgHere we can see 2 things.
The apparent noise level increased.
The gain configuration for the ADTG was identical. The signal level delivered to components further down the signal chain was very similar.
log2(14169/13702) = 0.048 EV
So we can conclude that the increased noise was from poisson statistics, and CMOS amplification.
We can also see that the FPN has not been (significantly) reduced. The increased shot/CMOS noise has reduced the apparent effect of the FPN, but the FPN level is very similar. I can attempt to use multiple exposures to average out the random noise, leaving the FPN component intact, however, if the FPN noise is not "
really correlated from one shot to another", then the level of FPN will also be reduced.
Since the FPN is reduced in
identical exposures with CMOS gain,
where the signal
through the ADTG stage is higher, and the level of FPN does not appear to differ (significantly), with reduced photons on sensor,
or,
directly with CMOS gain (ie: CMOS doesn't reduce the noise, it simply produces a knock on effect), it is my conclusion that the FPN noise is directly related to the ADTG stage.
This FPN noise
may be a result of the scales of the individual ADTG gains. Or it may simply be a fixed component of the ADTG stage at
it's noise floor.
Since we reduced the total noise of the ADTG stage with register adjustments, the FPN component of the ADTG stage is now more apparent.
https://theory.uchicago.edu/~ejm/pix/20d/tests/noise/index.html#patternnoiseThis gives an indication of how visually disruptive pattern noise can be -- even though the fixed pattern noise is only about 20% of the overall noise, it is quite apparent because our perception is adapted to picking out patterns, finding edges, etc.
If the FPN component can be reduced with register adjustments also, it may be beneficial to sacrifice total noise reduction (my DR is better then your DR), in order to reduce the FPN component. In other words, we reduce (the engineering definition of) DR, but we
increase the visual aspect of the output images.
This is what I was trying to explain in the dual_iso thread. Engineering DR has it's uses, but it is useless for describing what is happening
between (white) and (stdev).
My first guess (for reducing FPN), would be increasing the gain of a single component in the ADTG stage, ie: 888x or 0xFE or 8/9/A/B, and observing the effects. Then expanding on that by changing the gain of the individual registers in each ADTG gain stage.
Take it easy, the current state is research. As in, "If we knew what it was we were doing, it would not be called research, would it?"
