Why doesn’t full well capacity always match ADC bit depth × gain?
Asked 11/16/2020
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I’m comparing full well capacity figures from Photons to Photos/DxO with calculations based on ADC bit depth and gain at base ISO. For some cameras, full well capacity is close to (2^bit depth) × gain, but for others it only matches if I assume a different bit depth than the published ADC specification.
Examples:
- Nikon D3300: 12-bit × 8.41 ≈ 34,439, which matches the listed full well capacity.
- Olympus E-M10 II: 12-bit × 6.403 ≈ 26,220, also a close match.
- Canon EOS 2000D: using 14-bit gives about 96,905, but the listed full well capacity is about 24,226, which only matches if I use 12-bit.
- Sony A6000: using 12-bit gives about 8,268, but the listed full well capacity is about 33,083, which matches 14-bit instead.
Why would a camera’s listed full well capacity appear to correspond to a different bit depth than its published ADC specification?
Also, is it correct to think of full well capacity as being limited by whichever is smaller:
- the effective ADC range in electrons, or
- the pixel’s physical charge capacity?
And can the pixel-side factor be estimated from pixel pitch alone?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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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.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
5y ago
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Not necessarily. ADC bit depth tells you how finely the signal can be digitized, but it does not guarantee the sensor is delivering that many meaningful bits of real image information. A camera may use a 12-, 14-, or higher-bit ADC while the effective sensor precision and usable dynamic range are lower.
So full well capacity should be understood primarily as a physical pixel/sensor charge capacity, while ADC depth and gain determine how that charge is mapped into digital codes. Those two are related, but they are not always identical in practice, which is why your simple bit-depth × gain calculation can match some cameras and not others.
Pixel size does affect engineering dynamic range because larger pixels can potentially hold more charge. But bigger pixels also cover more area, and total exposure behavior is tied to light per area, so higher-resolution sensors do not automatically clip sooner just because pixels are smaller.
From the information given, you generally can’t derive a reliable universal “x” value from published specs alone. Pixel pitch by itself is not enough to calculate full well capacity accurately across cameras.
So yes: the usable limit is effectively whichever is lower—the physical pixel charge capacity or the digitization/mapping limit—but published ADC bit depth alone is not a dependable stand-in for true sensor capacity.
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