From: Jeffrey D. Mathias (jeffrey.d.mathias@worldnet.att.net)
Date: 10/29/01-07:29:57 AM Z
Sandy King wrote:
> ... However, since actual printing times are
> established by the time needed to print the first maximum black (or
> 90% of maximum black) it is my opinion that Dmax or 90% of Dmax
> should be the step that establishes the printing speed of an
> alternative process with any given light source.
Speed points can be confusing and may be defined differently for
different situations. However there are two pieces of important
information. One is the amount of minimum exposure to reach a desired
threshold on the characteristic curve of the material. The second is
the least amount of exposure that will provide the maximum black desired
in the print. These values can vary as selected by the photographer,
but should be consistent when employing good working technique. This is
simply called the film exposure or print exposure.
The first is selected by deciding on the curve portion to use for deep
shadows (like the toe or straight line). This is usually applied to the
negative and selection can be influenced by the subsequent printing
process. For Pt/Pd, placing all the exposure on the straight line
portion of the film negative generally gives better results by providing
more detail (local contrast or gamma) in the shadows. The portion of
the curve may or may not be some theoretical speed point, but it is the
point selected to perform as desired.
This film exposure value is also dependent on the film processing or
contrast control.
Another important consideration is the practical aspect of exposure
measurement. It is rather difficult for the eye to accurately access
absolute light values, and especially difficult to determine dark values
without texture. It is much easier to determine and select the darkest
portion of an image to have texture, so a value of Zone II or III can be
more useful in measuring and selecting exposure. This is also a
preference of the photographer geared toward what works for them.
If for example, one chooses to measure at Zone III and wants to use the
straight line portion of the film, they must make sure the exposure
gives them three zones of space on the straight line film curve below
that Zone III selection. One could just as readily measure at Zone VIII
and leave enough space on the curve. Keep in mind that changes in
contrast control can alter things.
Also keep in mind that any straight line portion of any film will have
density above base plus fog.
Printing exposure can be similar, but is usually determined to simply
set the threshold level of maximum black desired as available from the
materials and process. Basic printing exposure can be set as the
minimum exposure necessary to provide maximum black when printing the
film base plus fog. This will provide for the maximum dynamic range to
be extracted from the negative. Keep in mind that not only is the
definition of maximum black selected, but the dynamic range utilized may
be selected as well.
One must be cautioned to not simply set exposure to an arbitrary
percentage of Dmax as this will likely result in the loss of maximum
black or inconsistencies when varying materials. Also it is important
to realize that maximum black is not the same as Dmax of the material.
Every material I have used can produce a darker black (Dmax) than is
usable (maximum black).
With experience one should realize that there is no practical speed
point other than the exposure they select and determine to provide the
results they desire.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
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