So, if i understand well, in this way, the noise is our friend if we need less posterization because the 8bits depth ?
Correct.
To be effective for reducing posterization, the noise must be applied before the quantization (therefore, analog ISO noise is fine). Adding it in post will not help.
So it means that if we film a scene very illuminated outside with the 8bits , we have all interest to use a variable filter ND.
I disagree here. Lower bit depths were added to reduce the data rate at higher ISOs (where the lowest bits are mostly noise, and because of them, the image doesn't compress well).
I don't think filming with 8 bits is actually your goal. More likely, I assume you actually want to minimize the data rate. With uncompressed data, it's easy (all frames have the same size: W x H x bpp/8). With lossless compression,
you can't choose the compression ratio in advance; this is given - roughly - by the entropy of the image.
These "fake bit depth" settings (actually dividing the signal by a power of 2) are one way to reduce the entropy (by removing the least significant bits, because of integer division). That's a
lossy operation (going from 14 bits to any lower value).
After this step, compression is lossless (that is, the uncompressed data will be identical to what was fed to the compressor).
(That means, 14-bit lossless will give raw data
identical to uncompressed 14-bit (100% identical, not one single bit different, contrary to
some earlier report), while "12-bit lossless" should be interpreted as "14-bit to 12-bit conversion - lossy by definition - followed by lossless compression", so it should have the same number of useful levels as uncompressed 12-bit. Please note the 12-bit lossless is
not identical to 12-bit uncompressed - they differ by a constant value and possibly by some round-off error, and the same is true for lower bit depths.)
Back to entropy - increasing the ISO and underexposing (to keep the image brightness unchanged) will increase noise in the image (at all brightness levels). So, what you may gain by reducing the bit depth, you will lose from the added noise. Here's a quick test (static outdoor scene exposed to the right, plain 1080p, compression rates taken from ML menu):
Bits per pixel 14 12 11 10 9 8
ISO 100 1/100 61% 53% 50% 48% 46% 43%
ISO 200 1/200 62% 54% 51% 49% 47% 44%
ISO 400 1/400 63% 54% 51% 49% 47% 45%
ISO 800 1/800 65% 55% 52% 50% 48% 46%
ISO 1600 1/1600 67% 56% 53% 50% 48% 46%
ISO 3200 1/3200 70% 57% 53% 50% 49% 47%
ISO 6400 1/6250 76% 60% 55% 52% 50% 48%
ISO 12800 1/12500 79% 63% 57% 53% 50% 49%
=>
there's nothing to gain by using a lower bit depth at a higher ISO + underexposure.
Of course, if your goal is to record at 8 bits, no matter what, then of course, the best thing to do is to increase ISO at 12800 (or at least 6400) for half-decent results. It's also trivial to change the code to get 6 bits, 4 bits or even one single bit, if that's your goal
My advice:
first expose properly (ETTR, zebras, raw histogram, whatever you prefer), and
afterwards choose the optimal bit depth. Not the other way.
