[filmscanners] RE: Dynamic range

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

Roy,

>   "Dynamic range is the ability to distinguish tonal differences."
> If you take that as a given, naturally you're going to directly lead
> to the idea of Dynamic Range being equivalent to number of levels and
> that a 12-bit file has more DyR than an 8-bit file.  Simple logic.

That statement IS true, if the number of bits was not the limiting factor.
If you are digitizing 4 bits of noise into 12 bits, then they will digitize
the same dynamic range of the input signal.  A more accurate statement would
be that a 12 bit file can HOLD more levels than an 8 bit file.

> The numbers in the 8-bit file go from 0 to 255, in the 12-bit file they go
> from 0 to 4095.  These numbers are NOT signal values, in no way is the
> signal represented by the value 100 twice the size of the signal
> represented
> by the value 50.

Well, yes it is.  It's a direct reading of voltage from the CCD.  It has
nothing to do with density values, the human eye, logs or anything.  If your
A/D measure, say, 1V/count...then 100 IS twice as many volts as 50.

> They just plain AREN'T signal
> values.

Well, they are when they come from the A/D.  If you manipulate them in PS,
then, of course, their relativeness has changed.

> The ONLY numbers you can put into the DyR ratio are the
> measurements of real
> signal values.

That's true for the dynamic range of the scanner, but it's simply a fact,
that N bits are required to hold a particular dynamic range.  That in no way
assures that the data that's in those bits occupies the entire range of
values mind you.

> In imagining, a piece of paper with the maximum amount of
> black ink, outputs the least amount of light possible.  Again that amount
> of light is the MDS for that paper/ink combination.

That's not necessarily true.  You may ONLY be able to print a black of, say,
1.8D, but you can print tones in an increment of .01D.  Your max black in
this case is not the minimum discernable signal, it's merely a limit of the
ink darkness.

> And, Austin, yes I know in this case I'm saying density range is exactly
> the same range as dynamic range with note that the max density corresponds
> to the min light and the min density corresponds to the max light.

But I still hold that that is not correct.  I understand why you believe
that, but see what I said above about that.  You CAN have a piece of film
that has a DENSITY range of .4 to 1.8, and the minimum discernable signal
could be FAR less than your density measurement, in this case, the MDS would
be simply noise.  Again, density range says nothing about your ability to
discern within that density range, that, in THIS DISCUSSION is typically
limited by noise.

Austin

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