[filmscanners] RE: Density vs Dynamic range

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

> > Smallest signal could be either "smallest discernable signal", OR, could
> > be
> > "smallest signal level".
> >
>
> For film scanning, the smallest signal corresponds to dMax - the optical
> density of the film that transmits the smallest signal.

Hi Dean,

That isn't necessarily true.  There is absolutely no "standardized"
definition for every application of the term "smallest signal", even as it
relates to scanners.  It IS an ambiguous term.

> Going back to the
> ISO proposal, they state that dMax is the density where the
> signal to noise
> ratio is 1.  Nothing in the scanner standard indicates what to do if the
> scanner clips before reaching black - it appears that they are
> assuming that
> any reasonable scanner will not clip the black portions of the image.  The
> dynamic range of the scanner is calculated as dMax - dMin.
>
> The working draft for "Photography - Electronic still picture imaging -
> Noise measurements" seems to be more complete, or at least
> involved.  Within
> this ISO document they explicitly define, in section 6.2, the
> dynamic range
> as the ratio of the maximum unclipped luminance level to the minimum
> luminance level that can be reproduced with a signal to temporal noise
> ration of at least 1.

> This means that in a camera that clips before
> reaching a true black luminance level the denominator used for calculating
> the dynamic range is not the noise level.

Your missing how logs work.  You can take the noise from EITHER DMax or DMin
in their equation, and they SPECIFICALLY state that DMax IS noise.  That is
what YOU quoted here means:

"  Going back to the
> ISO proposal, they state that dMax is the density where the
> signal to noise
> ratio is 1. "

That IS noise, and noise in a scanner is at the DARKEST end of the scale
(DMax), not the lightest (DMin).  SNR = 1 means when the signal is EQUAL to
the noise.

> Much of the current "discussion" revolves around minor differences in the
> definition of  terms.

AND understanding what the terms mean.

Regards,

Austin

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