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

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

> Hi Tony,
>
> > Number of bits have two main roles. They do indeed represent the
> > theoretical
> > maximum density that a scanner could have if the electrical
> > components were
> > up to scratch and could use those bits to their full.
>
> Not true.  You can represent ANY density by any number of bits.  I can
> represent an entire density range with two bits:
>
> 00 - at or below dMin
> 10 or 01 - between dMin and dMax
> 10 - at or below dMax

BTW, that should be "at or ABOVE dMax"...sorry.

> This is a completely valid encoding of density range.
>
> I have to run now, so I'll comment on the rest of your post later.
>
> Regards,
>
> Austin

And...to add further to this, as I have a few moments...  Density values are
absolute values, just like one foot is an absolute value.  They have meaning
in and of them selves.  Someone decided what the exact length of one foot is
(within a tolerance of course), as well as density values.  They mean the
same thing everywhere...a density value of 1.6 means exactly that in La
Jolla CA, as well as in Pascagoula, MS.  Same with one foot.  But, the tonal
values out of a scanner are not the same everywhere, nor are they the same
even between scans!  The value 136 in La Jolla, CA...does not have the same
tonality as the value 136 in Pascagoula, MS.

Scanners are not calibrated TO anything, except themselves.  That is why the
data values you get from the scanner are not the same "density values".
There is no direct correlation between them, unless you were to calibrate
your scanner in the same way densitometers are calibrated (even that is
insufficient, as the sensors used in film scanners have different
characteristics than the sensors used in densitometers).

Now, WHY would anyone in the first place say that number of bits has
anything to do with density range...because they made some erroneous
assumptions.  Density range is stated in a ratio of "to 1" (:1), and a
density of 3.6 is 10**3.6:1 or 3981:1 (and someone, at some time, decided
what the density value of 1 physically is), which the number 3981 requires
12 bits to represent in the binary system, if you are going to represent
every integer value from 1 to 3981...BUT...that's the rub...a value of 3981
from the scanner is NOT the same as a density value of 3.6 (3981:1), for the
reasons explained above.  And, no, they are not close enough for government
work ;-)

Regards,

Austin

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