Shel,
Here are some simple dynamic range formulae: The analog/digital converter
(ADC) in a scanner puts out an integer. For a 16-bit converter, the range
is between 0 and 2^16-1 (65535). This range is typically linear, which
means that, for example, a value of 100 is twice as bright as 50.
For a linear ADC, the dynamic range is the ratio of the range (max - min) to
the resolution. For a 16-bit ADC, you can write it as:
(2^16 - 1) - 0
------------------ = 65535
1
For scanners, people quote the base-10 logarithm of this number:
log10(65535) = 4.8165
For a 14-bit ADC, the numbers are:
log10(2^14 - 1) = log10(16383) = 4.2144
These are the maximum POSSIBLE dynamic ranges with 14-bit and 16-bit
representations of the scanned values. They are upper bounds. Marketing
people love them because they are big and sound good. In reality,
electrical noise and distortion will reduce the true dynamic range
substantially, even for a 16-bit ADC.
Realistic numbers for desktop film scanners seem to be between 3.0 and 4.0
(at the highest). Really expensive drum scanners, like the Tango, get
closer to the theoretical upper bounds. That's why they're really
expensive.
I hope this helps to clarify the dynamic range issue a bit.
--Mark
p.s., yes, I do this kind of stuff for a living....
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