Julian wrote:
> there is a definite limit to dynamic range prescribed
> by the number of bits. An 8 bit scanner can never do
> better than a "Dmax" or ~dynamic range of
> log10 2^8 >= 2.4. This is because the lowest usable
> level "step" has to be around one LSB to be meaningful.
OK, I understand what you're saying and the reason for confusion is thinking
in linear terms not logarithmic terms. It *is* possible to make a scanner
with any dynamic range you want, but if it doesn't have enough bits to represent
the values or if it has too much noise, the amount of *useful* information
it produces will be minimal. It's possible to work around the 8 bit limitation
in an A/D by switching the input, but that's not the point of this discussion.
So OK, the number of bits determines the *useful* resolution.
Maybe the most useful thing which could be concluded from all this (which
I don't think anyone has stated - although I think Julian may have said
something similar): The number of bits in the output from a scanner determines
the maximum *theoretical* dynamic range of the scanner. Therefore if the
manufacturer claims a DR greater than the A/D can produce (of frankly even
equal to it), that claim should be considered with a very large grain of
salt.
So for instance if Nikon claim the LS2000 has 3.6 or more (log(2^12)=3.6),
it's dubious. The LS30 may be a special case since the A/D is actually
12 bit but the BIOS drops the 2 least significant bits.
Am I making sense now? :)
Rob
PS My apologies to those increasingly disinterested in this thread - I was
hoping I could find a useful, practical conclusion at the end of it. Now
that I have one, I will try not to pursue the pedantic details. :)
Rob Geraghty [EMAIL PROTECTED]
http://wordweb.com
