I think part of the problem with negative density range is that it is one of
those bits of math(s) that the Zone System seeks to hide. It is also
fortunately an area where we can - for all practical purposes - get away
with approximations - and personal interpretations so long as there is a
reasonable degree of consistency in these.
For once I would more or less agree with Scopick, except to say
that it is a property of a negative not _necessarily_ related to what we
want to see in a print. I'd interpret it as being the density difference
between the first printable shadow detail and the densest printable
highlight detail. Note that the first of these in particular may differ
slightly from process to process, and that the second is rarely printed at
the same exposure as the first!
Peter Marshall
Fixing Shadows and elsewhere:
http://faraday.clas.virginia.edu/cgi-uva/cgiwrap/~ds8s/Niepce/peter-m.cgi