At 10:21 13/01/01, Austin wrote:
> > > The pixel values (for which the range of is the
> > > theoretically highest Dmax for the scanner)
> > > are relative to each other, not absolute, ...
>
> > Correct ... the "pixel values" associated with measuring Dmax may be
> > relative ... but "Dmax" is a measured value, is absolute, and belongs
> > to film. Small point, but let's not confuse terms.
>
>The scanner manufacturers use Dmax as a specification item, which you said
>they didn't, but they do. We were talking about that, not a wit about film.
>We were talking about how many bits correspond to the different values of
>Dmax (amongst many other things), and that is NOT measured.
Like most specification stuff, nothing is clear cut and manufacturers adopt
shorthand methods of describing things - which is fine if everyone
understands and agrees. In a former life I wrote specs for radars and
processing systems, and wrote and assessed tenders for same so I have
participated at first hand in the gamesmanship of manipulating specs. This
explains why I am in my element here, and apologies to those who are not.
Scanner Dmax, for better or worse, is often used as a shorthand for
"Density Range" or "Dynamic Range". This doesn't seem too incomprehensible
or even reprehensible to me, since the figures must be close, because Dmin
is pretty close to "no film at all" . I mean...
Dynamic range (or density range) = Dmax-Dmin
where Dmax is the maximum film density that can be measured by the scanner,
Dmin the minimum....
Since clear film (fully exposed slide) is almost transparent, Dmin is
close to zero, so making an assumption that the scanner is set so that it
can just record Dmin (by adjusting exposure), then
density range = Dmax-Dmin ~= Dmax - 0 = Dmax
Cheers,
Julian
PS There is another issue that comes up here - I have assumed that Dynamic
range (which until now I would say is the same thing as density range) is
Dmax - Dmin where you measure Dmax and Dmin _with_the_same_setup_ - that
is, during the one scan.
Nikon may argue that their Dmin is measured with the exposure set low, and
Dmax with the exposure set as high as possible. This means that they can
get up to another 2 to 4 stops(!!!) into their claimed DENSITY
RANGE. Which might explain why they use the term Density Range and not
Dynamic Range - Dynamic Range certainly means the range that can be covered
without changing the setup i.e. the range available at one instant.
So I can see it is quite possible that Nikon MAY be able to argue that they
cover a Density Range of 3.6 for the LS2000 or 4.2 for the LS4000, although
you have to do a couple of separate scans to see it, which is not quite
what you would want and certainly not what people are assuming when they
read the spec.
The mere presence of exposure controls on the Nikon scanner tends to
support this idea.
So the LS2000 MAY in fact have a density range of 3.6, but it's Dynamic
Range could still be 2 (or is it 4) stops less than this - i.e. 3.0 or
2.4. Is it coincidence that most the measurements I have seen are in this
range, from memory about 2.6? (I assume people have been measuring Dynamic
Range, not Density Range).
Julian Robinson
in usually sunny, smog free Canberra, Australia
