I think the key is the same as in electronics and
elsewhere, namely understand what your instruments are
actually measuring.
From the 1920s through the 1950s a long series of
research reports was published by Loyd A. Jones, along with
some others, of Kodak Research Laboratories, on the tone
rendition of film and paper. Much of this was directed
toward black and white but has applications even for color.
One of the results of this research was the speed system
adopted by the ASA in the mid 1940s. Among other things
Jones wanted to find out the _minimum_ exposure that would
result in an _excellent_ print. The idea was that film
produced the sharpest images and least grain with minimal
exposure. Film is better now but this still is true. After
extensive testing on actual scenes and blind testing of
prints from the negatives, Jones found a definite speed
point for minimum exposure but also found that the
overexposure latitude was very great. That is, once the
film gets enough exposure to make a good print further
exposure does not change the tone rendition over a range of
many stops.
The Kodak Speed System adopted by the ASA, and indeed
all subsequent speed systems, assume a fixed gamma or
contrast index for the negatives. The idea is that the scene
brightnesses would be accurately recorded so that print
contrast would be determined by the contast of the paper.
The development of the Zone System essentially reverses this
approach in that it adjusts the contrast of the negative to
fit a fixed contrast printing medium, the idea is to insure
fitting all important scene brightness data on the negative.
Either system works. One reason Ansel Adams promoted the
Zone System is that he had gotten poor negatives for some
important pictures due to under-exposure. While the
latitude of _overexposure_ of most film is very great, the
latitude for _underexposure_ is no more than one stop and
maybe less.
Assuming one has printable negatives made by any
exposure system one encounters another problem: namely the
tone range of paper prints to be illuminated by reflected
light, is far less than either the range of brightness in
the original scene or the range on the negative. However,
the eye still expects to see something like the original
tone range, at least in the mid-tones. If overall contrast
is lowered to present a high-contrast (wide tone range)
scene on the print the result will be seen by the eye to be
grayed out, especially if the scene is something fairly
familiar. The approach that has developed over time is the
compress the highlights and shadows to some degree while
leaving the mid-tones alone. In chemical photograpy this is
done partly by choosing paper with an S shaped charistic, or
the use of masking, or simply manual burning and dodging. No
adjustment of exposure and development will replace this.
The best one can expect of a light meter is that it
will help fit the scene brightness range into the range that
can be recorded on the film.
The main difference between a reflected light meter and
an incident light meter is that the incident meter can not
measure subject brightness range. It _can_ measure lighting
ratio directly, which is often helpful where lighting is
under control. A reflected light meter can measure subject
brightness range. Both types of meters are useful but
again one must understand what they are measuring in terms
of what will be recorded on the film.
I am glad to see a few of us still follow this news
group.
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Richard Knoppow
Los Angeles
WB6KBL