At 14:53 30/08/02, David wrote:
>Does that mean you claim that density range and dynamic range are equivalent
>measurements of the same physical quantity?
Well yes and no. Density range is normally a property of a slide or piece
of film, or an image on a film. Dynamic range is normally a property of
some processing device, like a scanner in this case. If you have a slide
that can just be scanned by a scanner without the scanner saturating or
getting the black bits lost in the noise, then the slide's density range is
the same as the scanner dynamic range, in that case.
A scanner doesn't have a "density range", but it has a range of densities
that it can handle. The maximum range of densities that it can handle in a
single pass is its dynamic range. The maximum range of densities that it
can handle under any circumstances is it's "static" range, or max range,
sometimes called just Dmax by manufacturers. (Inaccurately, but we think we
know what they mean. Dmax is not a range, it is a figure. When they say
this, they are by implication assuming an upper limit of 0dB as the other
end of the range).
So if a slide's density range is greater than the scanner dynamic range
then the scanner cannot capture the whole density range of the slide.
I am using the terms as they are normally used. Both are measures of range
of densities. One is the range of densities actually or potentially on a
slide, one is the range of densities that a scanner can handle.
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, usually expressed
as a ratio. On an actual slide it is easy enough to pick the largest
signal (the lightest density) and the smallest signal (the densest area
which is just discernable against unexposed film background). 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.
Julian
Julian Robinson
Canberra, Australia
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