[filmscanners] Re: Density vs Dynamic range

["" <peter@softwell.co.uk>]

> >Dynamic range is, in our case, (dMax - dMin) / noise. Density range is
> >simply dMax - dMin. Dynamic range is the number of discernable values
> >within a density range (in our case). Density range is simply the max
> >density value you can get minus the minimum density value you can get.
>
>
> This is a definition of dynamic range that is at variance with the
> normal
> definition and VERY confusing.  Dynamic range is a RANGE, this idea
> that it
> is "divided by noise" is just strange.  It is NOT "a number of values",
> it
> is a RANGE, that is, a ratio.  And it is nothing *necessarily* to do
> with
> number of bits, in fact dynamic range existed for a long time before
> digital processing existed.  Where number of bits comes in is that for a
> given dynamic range there is a particular number of bits which is
> capable
> of coding all the possible levels in a meaningful way, without throwing
> away significant information.
>
> Dynamic range is no more or less than the range between ...
>
>          [the highest level / voltage / power (whatever you are
> measuring)
> that you can measure or detect]
> and
>          [the lowest level you can measure and detect].
>
> The lowest level as has been said many times is determined by the noise
> level, and the highest level is determined by the part of the system
> which
> first starts limiting or saturating.
>
> In my view, despite your (Austin's) statement to the contrary, the
> dynamic
> range is PRECISELY the measurable density range, in our context.

This is almost my understanding. I believe what Austin is trying to say is
that to increase the number of bits to the point where the step size is
lower than the noise value gives no more information. There is some
justification in this view, however, there are methods, in particular
multiple scans, where more information can be recovered by reducing the
contribution of the noise.

My main point of disagreement is in the definitions of the terms. In my
view the DYNAMIC range of the scanner is determined by the CCD, the optics
(in terms of the contrast available, the digital noise created by the AtoD
(roughly half the step size) and other noise generated in the circuitry.
What Austin defines as the dynamic range could be taken as the "goodness"
of the device, by I believe you can have a scanner with a high dynamic
range but may or may not be noisy.
The DENSITY range is that of the film (base fog to maximum black). If the
density range is less than the dynamic range than the output range will be
determined by the film only.  If the dynamic range is less than the
density range then the scanner cannot read all the information on the
film.

Peter, Nr Clonakilty, Co Cork, Ireland

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