Archived posting to the Leica Users Group, 2002/03/26
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On 3/26/02 9:35 AM, "Jim Hemenway" <jim@hemenway.com> wrote:
> Austin & John:
>
> What do scanner manufacturers mean when they say that their devices have
> a high DMAX?
>
> Are they claiming a high dynamic range, yet use the term DMAX?
Well, there's a very good question with a very surprising answer.
What scanner manufacturers *actually* mean by Dmax is that the A/D
converters in their machines capture so many bits of information.
For example, a scanner which can capture 12 bits is capable of representing
4096 distinct shades of gray (ignoring color for the moment). It's
*theoretical* dynamic range is therefore
log10(4096) - log10(1)
Which is 3 and a bit (sorry don't have slide rule handy)
Therefore the *only* thing most scanner manufacturer's Dmax figures tell you
is how many bits they capture. They tell you nothing *at all* about the
photo/mechanical/optical qualities of the scanner or its s/n ratio as a
physical system. They are *never measured*. They are just derived from the
number of bits of information the scanner returns for each pixel.
- --
John Brownlow
http://www.pinkheadedbug.com
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