# Full Well Capacity Questions

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!

• Bit depth has nothing to do with full well capacity. Low bit depth just forces a wider range of analog charges into each digital step. Nov 16, 2020 at 7:46
• Bill has the A6000 listed at 14bit. It could be that the A6000 is actually 14bit for single shot images and 12bit for everything else like the A6300 is; and Sony just never made that known. Or there could be an error. You could contact Bill and ask; he is quite helpful, and if there is an error I'm sure he would like to know. Nov 16, 2020 at 15:26