[filmscanners] RE: IV ED dynamic range... DYNAMIC RANGE!

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

Hi Todd,

> Well, I mean say it literally, as in DR= density range / noise,  ISO says
> Dmax - Dmin, but it does not define Dmin as density range, it
> defines it as
> Minimum density where the output signal of the luminance OECF
> appears to be unclipped. I just don't see how that is OBVIOUSLY a range.

I understand, and understanding it is a bit tricky, as I believe you've
stumbled on it too.  Their equation seems backwards, which was my confusion
with it at first, but when I worked it through, it works out fine.

The thing that is confusing, is obviously dMax is the darkest...but it's the
highest density number, and it's also the lowest energy...and the lowest
value out of the A/D converter that you're going to be able to read...that's
why it's the noise, as it is the lowest signal level.  dMin, same
thing...it's the lowest density number, but it's the highest energy as well
as the largest number out of the A/D converter.  The "full range" out of the
A/D will be from 0 to saturation (dMin)...so that's why I see dMin as a
range...because the scanner scans over a range, with dMin being the cap on
one end, and 0 on the other.

It appears they are assuming that the minimum output (1) of the "system" is
the same as the minimum discernable signal.  I believe that, because in the
spec, on p. 11, it says that the DR is reported relative to 1 as a ratio
value, which means you have to be able to resolve density values "to 1", so
1 is the minimum discernable signal.

This is the (and the only one) case when dynamic range equals density range,
simply because they are both log values, and both "terms" are the
same...both are a ratio of largest signal amplitude to minimum discernable
signal.  When the minimum discernable signal/noise isn't equal to the lowest
signal level, then the two aren't equal.  That does not mean they are the
same, but it means that they CAN be the same, for a specific circumstance.

Austin

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