Dynamic range and RAW file "bit depth"

Started Jan 25, 2014 | Questions thread
bobn2 Forum Pro • Posts: 51,034
Re: Dynamic range and RAW file "bit depth"

hjulenissen wrote:

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.

If you downsample the image (8MP is used in the DXO "print" number), I believe that zero-mean noise tends to average out, and some measure of DR might exceed that of a single sensel.

Which is sort of similar to the print-argument: when you have more spatial resolution than you need, this can be "re-invested" as better "intensity resolution".


Noise, and therefore DR is bandwidth dependent. In imaging, bandwidth is closely linked to pixel count, that is that the pixel count limits the sampling frequency and therefore the bandwidth. If you restrict the signal bandwidth (for instance, by low pass filtering) the noise power is reduced. The 'downsampling' think is a consequence of the low pass filter inherent in downsampling.

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