Dynamic range and RAW file "bit depth"

Started Jan 25, 2014 | Questions thread
Y Hafting
Y Hafting Contributing Member • Posts: 787
Non linear for sure.

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?

It is non-linear, but what data exactly is read from the sensor, is something only the sensor manufacturer or inventor could tell. This is why the raw converters from one manufacturer aren't made for processing raw files from others.

A strictly linear raw file would be hugely impractical. For example in bright, the noise is much bigger (in terms of variation in photons) than in dark areas- however we perceive darkness to be more noisy since our eyes are non linear. Thus it makes much more sense to have smaller steps in the shadow areas compared to highlight, and as far as i know this is how data is represented in every sensible format (jpeg, cr2, nef, dng, etc).

There is stricly no need for having the raw file storing the exact sensor readout either. Processing comes cheap compared to storing data, so in theory there is no guarantee that the raw file contains actual raw sensor data. The fact that the image size varies with the visible noise in canons cr2 format indicates that it is compressed.


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