[filmscanners] RE: Dynamic range -- resolution/levels

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

Roy,

> >> All the stuff about number of levels and resolution are
> artifacts of the
> >> digital process and not part of the DyR concept which existed
> way before
> >> the word digital was even coined.
> >
> ...
> > I believe the concept of resolution is inherent in the concept
> of dynamic
> > range.  Whether that "works" for you or not, at least for me,
> and for many
> > other engineers I know, is an important understanding.
> >
>
> Fair enough.  But I like to show why I believe that the "concept of
> resolution" that results isn't very meaningful.  See below.

It's SO meaningful, that EVERY PAPER showing how to calculate the number of
bits used does it based on dynamic range!  How on earth do you explain that?

> >> But the SIZE of the range is ONE number -- and it can be mathematically
> >> calculated with a subtraction OR with a ratio.  In the dynamic range
> >> case we always calculate the SIZE of the range with a ratio = max/min.
> >
> > I see how size can have a merit (which is a relative ratio),
> and range, as
> > they apply to dynamic range.  Size in the fact that the largest
> signal is N
> > times larger than the smallest...and range in that you can say
> "all integer
> > values from 1:1 to N:1".  BUT...realize that "all integer
> values from 1:1 to
> > N:1" really denotes a resolution over a particular "range"
> too...that you
> > have N discrete values.
>
> Yes, but I never said "integer".  In the real-world i.e. analog, there no
> reason why any real number couldn't be used.  What's wrong with going from
> 1:1 to 1.01:1 to 1.02:1 ...

Because noise is 1, and you can only measure in increments of noise.  In a
system that has noise of, say, 1V, you certainly can't measure 1.01V, now
can you?

> Here's why I have a problem with the "concept of resolution":
>
> Let me go through a simple example of a (semi-idealized) scanner.
>
> Here's the basic specs of the scanner:
> Density Range:  0D to 3.6D
> Bit Depth: 12 bits
> Number of levels: 4096
>
> A couple of simple observations:
> The density range is also 12 photographic stops -- each stop is .3 of
> density so 12*.3 = 3.6
> You can chop up the density range into 12 one-stop ranges i.e.:
> 0 to .3, .3 to .6, .6 to .9 ... etc to 3.3 to 3.6
>
> Photographically and human perception wise each of these one-stop
> ranges are equivalent in size.
>
> So now let's chop the density range into the 4096 levels.  The
> density range 3.6 divided by 4096 gives a little less than 0.001D
> per level.  Approximately, 300 levels for each of the 12 one-stop
> range.  Sounds like a great concept of resolution, doesn't it?
> We get a new level every 0.001D change in density -- it sure
> looks like a resolution of 0.001D.

But that's not how scanners work.  They know NOTHING ABOUT density values at
all!  They only know photons, and how many photons the CCD sees.  They see
relative values output PURELY AS A VOLTAGE (or possibly current), and that
voltage has a range, and has noise.  You can only measure as accurately as
noise, and as such, noise defines the resolution of that system.

<snip>

> Austin, don't take my word or the web's word for it.  Try it yourself.

Roy, I've designed film scanners, and have been designing digital imaging
systems for over 20 years.  I KNOW how they work.

All this stuff you wrote is simply irrelevant.  What ever the scanner does
with the data, or what the data actually represents WRT what the human eye
can see, or density values etc. has absolutely NO bearing on capturing the
data, and the DYNAMIC RANGE of the input signal, which is what we are
talking about.

You obviously need to capture the input signal, and what determines at what
RESOLUTION you capture it is the dynamic range of the input signal...nothing
else.  Specifically, the number of bits that you capture the input signal
should be such that you resolve down to noise...and you do that by
calculating the dynamic range, and, as I've always said, and said, and
said...you NEED so many bits to capture a particular dynamic range, period.

If it isn't the dynamic range of the input signal to the A/D that determines
how many bits you need/should use...what ever you want to call it, then WHAT
does?

Austin

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