Phil
It is worth remembering that the basic principles of densitometry are very
simple and easy to understand.
The difficulty is that many grossly overcomplicate desitometry and some,
but not all, followers of the zone system treat it almost as a religion.
Most books on densitometry that I have come across are very confusing for
the beginner.
As photography became more industrialised it needed standards on which
industrial processes could be based. It was necessary to be able
standardise systems to relate exposure to development for various
photographic processes rather than make 'seat of the pants' judgements.
When time and aperture are taken together we have the exposure. When a film
with a given exposure is developed the strenghth, temperature, nature of
developer and the length of development give different densities in the
film.
Hurter and Driffield worked out a system that took these factors into
account. In essence this was related to the brightness range that the
human eye can see; in camera terms this is about ten stops. This, in turn,
is related to the density range on the film from opacity to transparency in
ten stages. So, in practical terms, one stage, one stop.
These ten stages have numbers associated with them so that we can talk
about density and make calculations and prepare graphs to make comparisons.
So a number was thought of for the range of ten stops between completely
dense film, opacity, and clear film, transparency. It was 1000. But in
the days before computers and pocket calculators it was easier to do the
calculations using logarithms. The logarithm of 1000 is three, of 100 it is
2 , and of 10, it is 1. So number of noughts after the 1 gives the log.
For some reason, that mathemeticians understand, calculations using logs
enable you to add and take away to multiply. But for us using logs means
that each of the ten stops between opacity and transparency can be
expressed as 0.3, that is a tenth of three. Step wedges are produced that
show each of the stages. The most convenient are probably those made by
Stouffer where each stage, or step, on the wedge is half a stop or 0.15
with transparency as 0 and opacity as 21. (Judy got me a number of these
Stouffer wedges and I am very grateful to her).
We can compare the densities on the step wedge with those in the negative
so that for most purposes we do not have to have a densitomiter.
For develop out silver gelatine paper paper, where the chemicals bring
out a latent image from a short exposure, the film should only have a
density range, measured from first perceptible tone to densest area, of
about three stops which would be around 0.9, i.e. 3 x 0.3. The film base
should read less than a tenth of a stop ( 0.03 ). If the film is much
denser in the highlights, by the time the light from the enlarger has
managed to get through it to the film, the paper under the thinner parts
of the negative will have received so much light that it will block out so
that shadow detail will be lost. This means that all the information needed
for the final print has to be squeezed into those three stages or stops.
This is done by giving the film the exposure needed to get the shadow
detail but developing the film so that it does not become too dense. If the
film receives such a short exposure that the shadow detail does not affect
the film then no matter how long it is developed,that shadow will not show.
But if you develop the film for too long the highlights will become so
dense that that they will not print. This gives rise to the old saying,
from which the zone system is derived, expose for the shadows and develop
for the highlights. Another way to express this is to say the longer the
exposure, the greater the density : the longer the development, the greater
the contrast. as Peter Marshall has pointed out, by careful combination of
film exposure and development it is possible to record on film more than we
can perceive directly and then print that information onto the paper.
For many alternative processes there are other factors. The light sensitive
layers are so slow that contact negatives are needed. The processes are
either fully or partly printed out. In other words the light is made to do
the work rather than chemical development. When this happens, the light
through the thinner part of the negative affects the light sensitive layer
at the surface forming a mask that slows down the effect of the light on
the salts just under the surface so more time is allowed for printing
through the denser highlight areas of the negative. This means that the
negatives have to be more dense for print out processes whether they be
iron based like cyanotype or platinum or kallitype, or, as Judy has said,
just silver gelatine paper designed to printed out rather than developed
out. A cyanotype needs a range of about 1.4; a platinum print, to bring
out all the potential of the process, about 2.0 and a good carbon print can
be made from a negative with a density range of 2.6. This does not mean
that you have to have density range as high as these to make a good print.
At this stage such factors as the brightness range of the original scene
and the aesthetic judgement of the photographer or printer come onto play.
To get a full range of gradation on a gum print the negative should only
have density range of about 0.7 but more than one printing can be made to
increase the contrast. One print gum prints can be made but they lose in
the subtlety of gradation.
I hope that this helps
Terry King