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Re: Digital Negatives with enough Tones for Pt/Pd

On Fri, 7 Jul 2000, Jeffrey D. Mathias wrote:

> Oh, but dynamic range and bit depth ARE dependent on each other.

Nope!  That is simply not correct.

> To follow your analogy: one wishes to keep the number of stick sections
> per length close to equal because to have less sections per length would
> produce an unacceptable level of posterization (not enough sections per
> length).  Now, as a result, in order to get a greater dynamic range one
> must increase the bit depth, else they end up with too few sections per
> length.
> 
> Yes even a 1-bit image can have the largest dynamic range (pure black to
> pure white), but it is useless for producing a typical Pt/Pd print.

Notice that it is "your" criterion of keeping the size of the divisions
(sections) equivalent sized that depends on both bit depth and dynamic
range.  Having a condition that depends on two independent quantities
doesn't make these quantities in any way dependent on each other.

As far as the posterization issue you mention goes, I thought you wanted
to minimize this and were complaining about only being able to get 8-bit
scans/prints?  Basically, you want to get more than 256 levels.  That
really means that you want to increase the bit depth of your scan to get
more divisions.  The only reason to increase the dynamic range would be if
what you are scanning exceeds the dynamic range of the scanner.

To make another analogy.  Say you have an object that is around 2 feet
long and you'd like to measure it.  Using a ruler (1 foot long) would be
possible, but would require two separate 1 foot measurements.  This is an
example of wanting to increase the dynamic range (length) of the measuring
instrument.  If the ruler scale (bit depth) is fine enough to get the
measurement precision you wanted, you'd not want to change this.

Similarly you could very easily measure the two foot object with a yard
stick (3 feet long).  The question here is whether or not the scale (bit
depth) is fine enough to give you the precision you want?  In this case,
you have more than enough dynamic range.  In fact you could decrease the
dynamic range and still only need to make one measurement.  However, if
the stick is marked at one inch increments and you wanted 1/10th of an
inch precision you'd need to increase the number of divisions in your
scale (bit depth) to achieve this result. 

- Wayde
  (wallen@boulder.nist.gov)