Dynamic range and RAW file "bit depth"

Started Jan 25, 2014 | Questions thread
John Sheehy Forum Pro • Posts: 21,238
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.

That's an over-simplification. 0 is an important value, and many cameras have pixels "darker" than 0; in fact, black is actually recorded at something like 2048, so values of 2047, 2041, etc, are possible due to the bi-directional effect of read noise. Many cameras start out with data like this, and clip any value 2048 or lower to zero and subtract 2048 from everything else, and some cameras will stretch the RAW histogram out so that the highest number doesn't lose anything. So, your 0 to 16535 range is either non-existent, or it is actually scaled from something else.

The dynamic range of a sensor is the ratio between the brightest and dimmest recordable light intensity (or clipping and noise floor).

DR in system bound by noise is just an abstraction; not a concrete reality, as it would be if the black end were simply clipped well above the black level. Signal is detectable in digital sensors far below what their DxOMark DR would suggest; it is merely not distinguishable in fine detail individual pixels, because the variance of noise is much greater than these small signals. Taken as an average, however, over many local pixels, signals can become more distinct from noise. You can take any modern sensor, and photograph a checkerboard or chessboard with it, at signal levels where the brightest squares are stops below the so-called "noise floor", and if you know where the squares are in the RAW, and average over those squares, you will recover the checks. Same if you shoot a large white letter against a black background; the white could be exposed so that it is well below the "noise floor", and filtering of noise and/or image reduction can easily result in the letter being identified.

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.

This is because they are averaging neighboring pixels on sensors with more than 8MP to be virtual 8MP sensors, in "Print" mode. In "Screen" mode, they base their DR figure on the pure pixel statistics. In screen mode, however, there is no problem getting a DR greater than bit depth, even with a linear RAW, by a mathematical process because black frame noise can be lower than 1 RAW value. In fat, the D300 and D60 Nikon cameras (anf the Pentax K10D, IIRC) have less than 1 RAW level of read noise at base ISO, but of course, this leads to posterization of deep shadows. It is true, however, that a camera with linear RAW values, that had enough levels in its output to avoid posterization could not have a pixel-level DR greater than slightly less than the number of bits it has, because to avoid posterization, the read noise would have to be at least 1 RAW level, preferably greater than or equal to 1.3.

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