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     áòèé÷ :: Filmscanners
Filmscanners mailing list archive (filmscanners@halftone.co.uk)

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[filmscanners] RE: Density vs Dynamic range


  • To: lexa@lexa.ru
  • Subject: [filmscanners] RE: Density vs Dynamic range
  • From: "Shough, Dean" <dean.shough@lmco.com>
  • Date: Tue, 18 Jun 2002 13:17:50 -0700
  • Content-return: allowed
  • Unsubscribe: mailto:listserver@halftone.co.uk

> 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.  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.  It is the minimum level.  This is
reinforced when they talk about measuring the noise level for cameras that
clip the black level.  Also note that the dynamic range is the ratio
max/min, not (max-min)/min.  Also no logs or differences of log quantities
are involved.

Much of the current "discussion" revolves around minor differences in the
definition of  terms.  The scanner ISO definition of dynamic range is not
the same as the ISO camera definition.  The manufactures use a different
definition of dynamic range - they pick the definition that provides the
best looking number that they can possible quote in their specifications.

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