Sometimes with the aid of heroic burning and dodging in
printing or printing masks. One reason Adams got interested
in the Zone System was that he had problems with making good
negatives of some of the subjects he photographed.
    The Zone System is based on the idea that the contrast
and density range of the negative should be adjusted
according to the subject so that prints can always be made
on "normal" contrast paper. That sounds like it makes sense
but does thats not always so. For instance, the visual
contrast of the print must be acceptable to the eye.  The
eye judges contrast mostly by mid gray tones. So, it a very
high contrast scene is rendered with much lower overall
contrast it will not look natural but, rather, will seem
grayed down, even though there is a full range of tones from
maximum black to paper white and detail in all sections.
When a low contrast scene is made high contrast the eye may
accept it because the eye seems more tollerant of an
increase in contrast, but again, if the scene is a familiar
one, it will look wrong.
    Adams was also concerned with getting all the values in
a scene recorded on the film. Although its often thought
that the films of the time shouldered off at fairly low
densites the published curves for 1940's films show that
most of them had pretty a pretty good range of densities.
   Probably the most thorough work on tone reproduction was
done under the leadership of Loyd A. Jones of Kodak Research
Labs. He and his associates published many papers describing
their measurement of actual scene contrast and its
reproduction. Jones also worked out the Kodak Speed system.
This was later adopted by the ASA with some unfortunate
changes. The current ISO method, while different than Jones'
method still retains his idea that exposure should be
sufficient to get the dark parts of the image which are to
have detail far enough up the toe to have adequate contrast.
Jones' criteria, based on tests with hundreds of prints of
many differing subjects, was that the minimum contast point
should be no less than 1/3rd the gamma (contrast) of the
straight line portion of the film characteristic. He found
that images with less exposure were judged to be inferior
while an increase in exposure did not make any difference
over a range of a great many stops. In effect he found that
there was a certain minimum exposure needed for good quality
but that once this was reached increased exposure made
little or no difference even though the negatives might be
dense enough to be hard to print.
   Because materials of the time showed increased grain and
image spread with density (image spread leads to lower
sharpness) he decided to base his speed system on the
minimum exposure for good shadow detail.
   All negatives were developed to the same contrast. The
contrast was such as to result in a gamma of about 1.0 on
the mid tones of the print. Reflection prints have a
relatively limited range of tones from minimum to maximum,
they can not, if viewed normally, have any tone brighter
than the ambient illumination. Transparencies can be back
illuminated or projected resulting in a much longer range of
reproduced brightness.
   The usual method of the time (and more so now with VC
paper) was to make negatives of "normal" contrast and adjust
the paper grade where the nature of the scene required
something other than normal toner rendition.
   Where the tone rendition must be distorted, that is
different contrast values in shadows, mid-tones, and
highlights, it must be done by burning and dodging or by
using a mask to accomplish the same purpose. Simply
adjusting negative contrast will not do it.