> (I'm ignoring colour for simplicity's sake).
Color isn't relevant. The sensor doesn't have any color information, only
intensity information. The color is deterministic...ie, a particular sensor
has a particular color filter over it.
> The real minimum and maximum
> light intensities which the circuit can resolve into a digital number is
> determined by the analog CCD circuit NOT by the number of bits used in the
> ADC.
True. And therefore this 'range', as it is typically called, can be
represented by one bit or 1000 bits. Which, I believe, was the point that
someone else was making...
> The number of bits in the ADC *only* determines the number of steps
> between the minimum voltage it can resolve into a number and the maximum
> voltage it can resolve into a number.
Basically, true. Typically, that 'number of steps' is called the
resolution, ie, if you have an 8 volt range, and you have three bits
(therefore can represent 8 different values), your resolution is 1V.
> The only reason I can see that a greater number of bits would help is that
> when you are at the extremities of the CCD's range, more bits should help
> resolve meaningful data from noise, or by reducing the size of the steps,
> reduce the loss of image information which lies between the steps at a
lower
> bit depth.
I don't follow you here.
> Ultimately, it's the CCD circuitry which determines the minimum and
maximum
> light intensities that the scanner can (in theory) resolve, not the number
> of bits used to convert it to a digital image
Agreed...
> which just determines the smoothness of the conversion.
I know what you mean, but that's the first time I've heard it called that...
> What Ed's demonstration of a relationship between bits and dynamic range
> demonstrated to me was that the numbers for some scanners simply *seem*
> to be the theoretical maximum determined by a mathematical relationship
> from a given bit depth *not* a real measurement of the DR as determined
> by the kind of tests Dave Hemingway described.
I was saying somewhat the same thing, but apparently for different reasons?
> In fact, referring back to my argument above, there's no reason why an 8
> bit per channel scanner couldn't have a dynamic range of (say) 4 if the
> analog circuitry is capable of measuring that range of light intensities.
No, that's wrong (and you were doing so well ;-). Dynamic range IS
resolution over ANY range, and 8 bits won't give you a DMax of 4. In fact,
8 bits is 48db, or a DMax of log10(2^8) or 2.4. Perhaps you are confusing
the meaning of DMax? The 'capture' range (range from min voltage to max
voltage), as you have aptly stated, is completely separate from resolution
(within that capture range).
Just a note on 'resolution'. Most people consider resolution to be, say,
the number of pixels wide the scan is times the number of pixels long the
scan is. That is true (though only by common use, not by correct use ;-),
but there are other resolutions. Consider that X and Y resolution. There
is also Z resolution, which is the bit depth of the pixel, more bits is
higher resolution. That is the resolution I am talking about in this
discussion, not the X by Y resolution of the image.
