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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

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
Tokyo, Japan

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