Looking at camera comparison sites today, I encountered statements like "color depth: 23.5 bits".

(See here for example, which gives the 5D Mk III a 24-bit colour depth, and the D7000 a 23.5 bit colour depth).

My usual understanding is that in a 24-bit colour representation, there are 8 bits per channel (i.e. 8 bits for each of the red, green and blue channels). Given that a bit is indivisible, how do I interpret a fractional bit depth? It this just somebody taking log2(max-color-value)** and forgetting to round/truncate, or is it meaningful in terms of hardware or image encoding (or something else)?

** e.g. if the actual sensor colour space is smaller than a 24-bit RGB colour space, and then the effective depth is log2 of that. But the pixels are 14-bit in most high-end cameras, so this doesn't make sense either...


Snapsort is using DxOMark for its sensor data, and DxOMark explain here what each of their scores mean. Specifically for Color Depth, they say:

Maximum color sensitivity reports, in bits, the number of colors that the sensor is able to distinguish.

i.e. DxOMark claim that the 5D Mark III can distinguish 2^24 = 16.8 million colours, while the D7000 can distinguish 2^23.5 = 11.9 million colours. Don't worry too much about the absolute numbers, all it means is that the 5D Mark III can distinguish about √2 times as many colours as the D7000, at least by DxOMark's metrics (which they don't make public).

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    Congrat's on 10k :) Oct 6 '16 at 14:49

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