Dynamic range and RAW file "bit depth"

Started Jan 25, 2014 | Questions thread
xpatUSA Forum Pro • Posts: 13,694
Re: Dynamic range and RAW file "bit depth"

szhorvat wrote:

Most cameras can record the sensor data with at most 14 bits of resolution. This means that the ratio of the lowest and highest representable values is 2^14.

The dynamic range of a sensor is the ratio between the brightest and dimmest recordable light intensity (or clipping and noise floor). I notice that DxOMark lists several sensors as having a dynamic range larger than 14 EV, e.g. the Nikon D800 has 14.4 EV, which corresponds to a ratio of 2^14.4.

How can they measure a dynamic range higher than the resolution of the sensor readout? If the sensor response is strictly linear, this shouldn't be possible.

So is the answer that the sensor response is not linear? Is this nonlinearity inherent to how the sensor works, or is it a designed feature aimed at increasing the dynamic range?

I'm asking this for my own edification, not because I need the information to accomplish anything.

Perhaps it would be best to start at the sensor, that is to say the well capacity expressed in electrons. One of my cameras has a stated well capacity of 77,000 electrons (e-) at saturation. This is just over 16-bits worth which, as you know, is 65,536 (including 0). So my sensor could have a dynamic range of log10(77000/1)/log2 = 16.23 EV, all other things notwithstanding . . . . .

Now, suppose I throw in the stated noise floor of 70e-, then my sensor real dynamic range is more like 10 EV or so.

If I now present my sensor output with it's 10 EV range to an 8-bit ADC is the sensor dynamic range any different than if it is presented to a 14-bit ADC? My point is that ADC resolution does not affect sensor dynamic range, IMHO.

Disclaimer: I don't go to DXOMark very often.

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