I was hoping someone that really knows about this would answer, but it doesn't
look too promising. So, I will suggest that you take a look at the book:
Davis, Phil, Beyond the Zone System 3rd Ed., Focal Press, 1993. I think that
what you are looking for can be found on page 29. The problem is that just
picking a high density value and a low density value and taking the difference
is assuming that the film's characteristic curve is linear which is not the
case. The density range is also not the whole story. You actually need to know
the relative level of exposure that gives you a certain density in order to
understand how a given subject tone will be recorded on the film.
In order to simplify all of this, we talk about the gradient of a linear
approximation to the fairly straight part of the characteristic curve. The
problem of course is picking which two points you want to draw the line through.
According to the Davis book there are at least three defacto standards. The
oldest being to define the curve slope as gamma. As I understand this you
define the density range as the maximum and minimum densities of the film and
gamma is the ratio between this density range and the exposure range needed to
give these densities. Ilford supposedly uses what is called the average
gradient method. In this case, the first point is chosen at a density of
0.1-over-base+fog. The second point is found by striking an arc with a radius
of 1.5. The intersection of this arc with the film's characteristic curve
defines the upper density point. Kodak uses what is called the Contrast Index
method. The Davis book doesn't define this, but says that it can be
approximated using the same technique as the average gradient method using a arc
radius of 2.0. I think I have a reference to the papers that define the
Contrast Index technique in another e-mail here somewhere if you want it. I
haven't had a chance to look into this yet.
I do think that since you asked this question the Davis book would be worth
taking a look at.
Hope this helps a little bit.
- Wayde Allen
(74160.3410@compuserve.com)