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...