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[filmscanners] Re: Dynamic range




"Todd Flashner" <tflash@earthlink.net> wrote:
>>>>>>>>>>>>>>
Well I guess this really breaks down to the simple difference as to whether
DyR is a resolution vs a range. I think your opponents agree that IF it were
a resolution then your approach would be correct, they just believe it is a
range, as its name would indicate.
<<<<<<<<<<<<<<

My latest theory as to what's the problem is that people don't understand
what the values reported by a scanner mean. What a value n means is "the
range of actual densities the scanner reports as n."

Consider a 1-bit scanner. The value "0" means "I saw a level somewhere
between true black and the first value I can recognize as "1", and the value
"0" means "I saw a level somewhere between true white and the first value I
can recognize as "1". Thus, if we view the value as the center of the range
it corresponds to, "0" does _not_ mean "black", it means a somewhat dark
gray, and "1" does _not_ mean "white", it means a somewhat light gray. So
dMax for this scanner is dark gray and dMin for this scanner is light gray.

When we view or print, we (incorrectly) interpret 0 to mean perfect black
and 1 to mean perfect white. But that's not what the scanner means. The
scanner means "I saw a value somewhere in a range."

Note that this range for a 1-bit scanner is incredibly wide: it reports the
same value for fully half the possible density ranges on the film. This is
called quantization error.

>>>>>>>>>>>>>>
Okay so in my example above, the dmax of the scanner coincides with the
densest patch on my target film that the scanner can distinguish over noise.
I said it to be a density of 3.7.
<<<<<<<<<<

And as the noise (including quantization noise) goes down, this value goes
up.

>>>>>>>>>>>
This corresponds to my film base+fog. Let's assume it could theoretically be
zero density (maybe the film doesn't fill the film holder).

So what don't I get?
<<<<<<<<<<<<<<<<

Similarly, the noise (including quantization noise) defines this level as
well. Note that this isn't film base + fog. It's film base + fog + noise.

>>>>>>>>>>>
If the density of the film base+fog were 0, and the readable dmax  occurred
at a film density of 3.7, using THIS spec, how could the DyR be anything
other than 3.7 - 0 = 3.7 for BOTH scanners?
<<<<<<<<<<

(1) The density of the film base+fog+noise will never be 0.
(2) The dynamic range is the _ratio_ of these values.
(3) An important noise component is _quantization error_. If you have enough
bits, it can be ignored, but if you don't, it can't.

David J. Littleboy
davidjl@gol.com
Tokyo, Japan


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