Full Well Capacity Questions

Question

As I understand it, there are two potential limiting factors on full well capacity.
a) adc bit depth and gain,
b) pixel size and ability to absorb photons
Whichever is the smaller value (in electrons), is limiting full well capacity.
Now I test a few different cameras. Data is found at www.photonstophotos.net under the headings, (i) "DxO Mark Gain Analysis" and (ii) "DxO Mark Derived Sensor Characteristics Chart and Table" - the former, providing gain values at each iso, the latter providing a full well capacity value. Data for adc bit depth is found at DxO website in camera specifications "bits per pixel".
In a few examples we get a clear match between the two.
Eg1. Nikon D3300, 12 bit adc, 8.41 gain (lowest ISO). So, (2^12)*8.41 = 34,439. This is very similar to the value given at (ii) = 34,446
Eg2. Olympus E-M10 Mark II, 12 bit adc, 6.403 gain (lowest ISO). So, (2^12)*6.403 = 26,220. This is very similar to the value given at (ii) = 26,228
But there are examples where things vary wildly.
Eg3. Canon EOS 2000D, 14 bit adc, 5.915 gain (lowest ISO). So, (2^14)*5.915 = 96,905. This is very different to the value given at (ii) = 24,226. BUT if we change adc to 12 bit = 24,222 value is now very similar. Why would the full well of this camera be listed according to 12 bit adc when it has a 14 bit adc?
Eg4. Sony A6000, 12 bit adc, 2.019 gain (lowest ISO), So, (2^12)*2.019 = 8,268. This is very different to the value given at (ii) = 33,083. BUT if we change adc to 14 bit = 33,077 value is now very similar. Why would the full well of this camera be listed according to 14 bit adc when it has a 12 bit adc?
Meanwhile, none of the above yet takes into account b) pixel size and ability to absorb photons. My understanding is that full well capacity from this perspective = pixel pitch (microns)^2 * x, where x is a value that describes the ability of the silicon (pixel) to absorb photons, and is typically between 1000-2000. My question here, is there anyway to calculate x based on given specifications or data?
And is it correct to say, that if b < a, full well capacity = b, and if a < b, full well capacity = a?
Many thanks!

Steven Kersting · Answer

Just because a camera is using a higher accuracy (bit depth) ADC doesn't mean the sensor is generating an equivalent accuracy (bit depth) of data. E.g. AFAIK there is no digital camera that exceeds 8bit/color accuracy. And processing 8bit color through a 14bit ADC and stuffing it in a 16bit file doesn't change anything... it's still 8bit accuracy.
Pixel size affects the engineering dynamic range. And a larger pixel has a greater potential because it has a larger capacity and difference between min/max. However, it also occupies more area. What matters in terms of recorded DR is light/area capability; and when you have more pixels of smaller size the light is simply divided among more of them. I.e. a sensor doesn't clip to overexposure earlier due to being of a higher or lower resolution.
There are some potential differences... more smaller pixels means more boundaries between them which could reduce fill efficiency; but this is an extremely small factor for modern sensors. And smaller pixels could have lower sensor read noise; and they have a lower gain requirement which works in their favor. In general you will find sensors of higher resolution, and of the same size, have equal or greater dynamic range for those reasons. In fact, that is why the latest sensors are going to dual gain designs... a smaller gain/full well capacity for lower light conditions and a larger gain/FWC for brighter light conditions.

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