Dynamic range and RAW file "bit depth"

Started Jan 25, 2014 | Questions thread
RussellInCincinnati Veteran Member • Posts: 3,201
bit depth affects PRECISION, not dynamic range so much

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.

No, it just means that whatever the scene dynamic range might be, that range of brightnesses will be recorded in the raw file using no more than 8192 different numbers. Perhaps the stored values record scene brightnesses with a higher degree of precision at higher brightness levels, see below.

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.

Then hopefully the Nikon D800 raw file format lavishes many more of the 8192 different numbers on representing the upper brightness range with precision, than the format dedicates to representing the darkest parts of the scene. The firmware designer could declare that the camera will store scene exposure values ranging from 13.5 to 14.5 EV in the raw files using all the numbers between 4096 and 8191, obviously with great precision, for example. And reserve the lower raw format values to represent all the scene brightnesses below 13.5 with far less precision, etc.

One could say that a 17-bit raw format (stored numbers each ranging from 1 to 128000) is needed to "perfectly" record the 80 000 or whatever different possible photon counts that can emerge from a commercial portable camera sensor. But realistically, the cameras can do a nice job of representing the scene with a mere 8192 different values. Judiciously representing the bright, relatively noise-free parts of the scene with more closely-spaced stored values, than are alloted to representing the dark parts of a scene.

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