[filmscanners] RE: Dynamic range

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

Todd,

>>> Does that mean you claim that density range and dynamic range are
equivalent
>>> measurements of the same physical quantity?

>> Well yes and no.

> This sounds like an amazingly lucid explanation.

I believe it misses THE important point as to WHY they are not the same.
DENSITY is a specific measurement, that is based on known samples...so you
have a density of 3.0 here in the USA, and it's the exact same density of
3.0 in Tanzania.

Density range is strictly a statement of either two values, the minimum
density and the maximum density, or an overall range (between two
densities).  That says NOTHING about what happens over that range, between
the endpoints.  Not a single thing.  Dynamic range SPECIFICALLY is a
description of what happens OVER "a" range.  It says nothing about what the
endpoints are, or what the overall range is (in physical terms).

I believe the confusion arises in the misunderstanding that just because
they are both stated in log math, that it is mistakenly believed they are
somehow related.

> > Dynamic range is normally a property of
> > some processing device,

I'd say it's more a property of A signal, and a device is made TO handle the
dynamic range of THAT signal.

> > A scanner doesn't have a "density range", but it has a range of
> densities
> > that it can handle.

Correct...but doesn't that contradict what you just said above?

> The maximum range of densities that it can
> handle in a
> single pass is its dynamic range.

Out of curiosity, why do you limit it to a single pass?  The dynamic range
of the scanners capabilities ARE the dynamic range of it's capabilities,
whether it's three passes or 1000 or 1.  There are multi-ranging techniques
that can be used to expand the dynamic range of A/Ds that have less bits
than you need to characterize the dynamic range you want to get.

> > You *can* talk about the "dynamic range of a particular slide"
> and be kind
> > of correct.  Or you could talk about the dynamic range of the
> medium (that
> > is, the particular film).  Dynamic range is, as it always has
> been, nothing
> > more than the range of largest signal to smallest signal,

No, it's the RATIO of the largest signal to the smallest.  Saying it is a
range is inaccurate, unless you are talking about log values, and even then
I disagree, as it is still simply a CHARACTERIZATION OVER a range, it is NOT
a range in and of it self.

> > usually expressed
> > as a ratio.

But a ratio does not make it a range.  If I have 100:1 the number of rocks
in my head than you do, that does not give me a range, only a ratio.

> > ...For the
> > medium, the relevant figures are the lowest POSSIBLE density, and the
> > highest POSSIBLE density that can still be discerned from background
> > black.  If you use the language this way, then the slide's
> dynamic range is
> > the same thing as its density range.

I don't see how that shows they "are the same"...because the slide has
PHYSICALLY measured values, of say, .02 to 3.6.  It says NOTHING about how
many values may be discerned in-between.

Austin

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