>The scaling I mentioned is universally accepted as a way of defining the blacks and whites.
That's what I said as well, though I referred to it as luminance values.
However, that has nothing to do with actual dynamic range of sensors or film, it is just an abitrary measurement.
In fact, it is possible to have far more DR in an image than will fit the scale, (ie, such as what one might obtain with an HDR). In that case, the DR is compressed and tone mapped to fit to the 0-255 scale.