Digital BW, The Print

Austin,
 
let's see if I got it right: the DR tells us how deep the scanner can see
into the film, and so higher DR values mean higher ranges of the  analog
signal strength we get from the CCD. On the other hand, bit depth is the
number of bits that the analog to digital converter (I suppose there must be
one somewhere) uses to map that signal. Given that the analog signal is
continuous, any number of bits we pick will be a limiting factor in that it
will not represent all the analog values (theoretically infinite), but this
is not a problem due to human eye limitations. We have a real limiting
factor when we use a number of bits which is not sufficient to represent the
ratio between the highest and the lowest analog value (which I think is the
actual definition for DR): so if we have an analog signal ranging from 1 to
10 (silly numbers, I know) and only use 3 bits (0-7), we lose some
information, whereas if we use 4 (0-15) we don't. Is this correct?
I don't know how scanners are designed, but I guess that once you have an
analog signal from the CCD (whose range defines the scanner's DR), you could
use different A-D converters: therefore the bit depth (defined as the number
of bits we translate the signal into), would be independent from the DR.
Does this make sense?
 
Alex
 

-----Original Message-----
From: Austin Franklin [mailto:darkroom@...]
Sent: mercoledì 26 settembre 2001 07.39
To: DigitalBlackandWhiteThePrint@yahoogroups.com
Subject: RE: [Digital BW] Bit depth, was Minolta DiMAGE Scan Multi PRO



> > If the range of the film stays fixed and the bit depth goes up the
> > width of the raw scan shouldn't change. If you decrease the bit depth
> > to less than the range of the film then the raw scan would get
> > narrower?
>
> Martin,
>
> I believe this is confusing because Austin has been trying for a long time
> to make dynamic range a function of bit depth

This is a fact. Please.  Bit depth IS a limiting factor of dynamic range in
the way current scanners are designed.

> Allow me to quote Andrew Rodney (digitaldog.com):
>
> "Dynamic range has nothing to do with bit depth!

He is right, and I have said this too, that you can REPRESENT any dynamic
range with two or more bits...but as I have said, that is NOT how scanners
happen to be designed.  And for a very good reason.  CCDs output an analog
signal that is (more or less) proportional to the light that the sensor
elements see...and when the light is twice as strong as the another light,
the signal has twice the strength!  AND that happens to pretty much coincide
with "integer density ratio values".  This is just REALLY simple.

> They are completely
> different spec¹s. You can have a scanner with 16 bits per color and a
> dynamic range of 3.3 and you can have a scanner with 12 bits and a dynamic
> range of 3.8. Bit depth is the number of steps. Dynamic range is
> the height
> of the star case. You can have a staircase that¹s 20 feet high and have 40
> steps. You can have a staircase that¹s 30 feet high and have 30 steps."

He is talking about theoretically, and I have stated this exact case too
time and time again, but is had nothing to do with the reality of how
scanners ARE designed and how they work.  A two bit scanner, though it could
REPRESENT a dynamic range of say, 4.0, could only give you line are!  You
would get NO tonality.




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