[filmscanners] Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?

["Austin Franklin" <darkroom@ix.netcom.com>]

> >Another misconception...though equally as common...the number of bits the
> converter
> >has, have nothing to do with the scanners ability to capture any
> particular
> >density range.
>
> Just out of curiousity and in simple layman's terms, what do the number of
> bits that the converter has  have to do with if not the density range? How
> does it impact on what is captured?

Hi Laurie,

First off, any density range can be represented by any number of bits (2 or
more).  Because you have 100 bits, doesn't mean you automatically can
represent a density range of X...and because you have two bits, doesn't
limit the density range these bits can represent.

What number of bits give you is DYNAMIC range, NOT DENSITY range.  They are
NOT the same, no matter how hard people try to claim they are, they are not.
They are two entirely different things.  Density range is merely the dMax
and dMin, it has nothing to do with how finely you can discriminate (resolve
tones) within that range.  Kind of like the difference between a 3 foot
ruler with one foot markings...and a 3 foot ruler with 1/4" markings.  They
both measure the same RANGE, but not to the same accuracy.  Dynamic range is
the ability to discern within the overall range, really the resolution.

If you want to discern 256 things, you NEED 8 bits...but that has nothing to
do with what those 8 bits represent...as they are simply relative unto
themselves.

> It has always been my understanding, rightly or
> wrongly, that the
> higher the number of bits the more detailed or refined the informational
> date captured from the original that is transmitted as data in the digital
> file with respect to highlight and/or shadow detail

That is correct....providing the system can actually take advantage of those
bits.  If you have a 24 bit converter, and 12 bits of it is useless (noise),
then what good are the 24 bits?

> with the density range
> figure represetning the range of contrast that can be captured.

And...that is correct too.

> In other
> words, "dynamic range" representing the contrast range of the capture's
> capabilities,

No, DENSITY range represents the contrast range...as you said just above...

> while the bit depth represented the quality of the data
> captured within that range particularly the extremes.

Which is DYNAMIC range.

Where apparently the confusion has arisen from, is the fact that density
range values are stated in relation ":1" as in "to 1", so a density of 1.0
is 10**1 or 10, and means "10:1", so people simply assume that the
resolution of the system is relative to some "1", which it is not
necessarily.  The values out of the A/D are not calibrated in any way, shape
or form to density ratio values...they are simply a relative measurement in
and of themselves.  You could, of course, design a film scanner and
calibrate it so it reads "accurately" like a densitometer...but none do.

As an example, your B&W 256 value (8 bit) file, can "represent" a density
range captured from B&W film of say, .1 to 2, which is a density range of
1.8 or 10**1.8 or 63:1...yet, you get different values for 256
tones...therefore, there, obviously, is no relationship between 8 bit
values, and the actual density range of the original film.

Regards,

Austin

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