A sensor cell can hold say 30000 - 60000 electrons. Larger pixels hold more electrons. So upper limit is 60000, which correspond to 16 bits. Now, this signal needs to be read out and converted to binary digits. The readout has some noise. Some CMOS sensors do it on chip, and they can have as low noise as 1-2 electrons, CCD-s have much higher readout noise. more like 15 electrons.
So the calculation is:
MF maximum signal is perhaps 60000 electrons, readout noise is 15 electrons. SNR (Signal Noise ration) is 60000/15 -> 4000 -> 12 bits.
DSLR (Nikon D3X)
Maximum signal 30000 electrons readout noise 2 electrons, SNR = 30000 / 2 -> 15000 which is about 14 bits.
So in this case the Nikon would actually be 14 bit while the MF back would be 12 bits. The figures are approximately in the ballpark.
Would MF backs make good use of sixten bits they would also offer excellent high ISO capability
That's about right for the MFD system, but even though I know you're using ballpark figures, you're still somewhat off on the D3X for two reasons:
1) It's not as good as say a Canon 1d MkIV in terms of minimum readnoise; the D3X is closer to 4 electrons than 2. That in itself is a drop of 1 bit of DR.
2) Your D3X DR calculation assumes that the minimum readnoise is attainable AT THE SAME TIME as the full pixel capacity (well depth) is also attainable. But because of A/D converter noise, the lowest readnoise in DSLRS is usually only reached at around ISO 800 or 1600, at which setting the maximum signal is reduced by around 4x - 16x, depending on the base ISO.
So the calculation you did correctly gives the sensor's
inherent DR, but fails to take the rest of the real-world camera system
into account (the A/D contribution to noise). You are in good company: I've seen people quote from Roger Clark's DR tables and plots
while missing the crucial nuance that these are sensor
values, and I know I've done it wrong myself in the past
. It's rather like saying that "because I can run a short 100m race at 5m/sec, and because I can also run a marathon, then I can run a marathon at 5m/sec" ["because I can readout a truncated max signal at very low readnoise, and because I can also store a max signal up to the full well depth, then I can readout the full well depth at very low readnoise"]. It's combining performance specifications taken under different, mutually exclusive circumstances. I guess it's an easy mistake to make, because people like you and I grew up on the strict engineering definition of DR=FWC/RN, which is fine for CCDs as the readnoise rarely changes with ISO, and ISO settings are usually only "flags" which do not actually decrease the maximum signal (...and scientific CCDs have no concept of ISO in the first place!). But that definition needs to be adjusted for CMOS sensors with real ISO, max signal and readnoise variations.
Just now Doug reminded us again that "it's the system that matters", and he's absolutely right; but I think that he is underestimating the importance of the A/D converter as one of the kingpins of performance.
While Nikon did make a breakthrough with the D3X, managing to greatly reduce A/D noise so that it was much less of a limiting factor at lower ISOs, there still is a modest trend with ISO (D3X on Sensorgen
). More recent cameras from Sony and Pentax are also greatly diminishing the trend of readnoise with ISO, by beating down the A/D component of readnoise. This is how the Pentax K-5 took everyone by surprise with its ~14 bits DR ( K-5 on Sensorgen
, and likewise the Nikon D7000.