From: Jeffrey D. Mathias (jeffrey.d.mathias@worldnet.att.net)
Date: 03/14/01-07:21:33 PM Z
Shannon Stoney wrote:
> ... I wonder if in the future, densitometers will be considered standard
> equipment the way light meters are today? ...
There is an important distinction to consider here. The eye is very
poor at judging absolute light intensity, but excellent at relative
comparison. However since the sun's output is consistent, photographers
have been able to accurately judge exposure based on their experience of
time of day, time of year, weather and such. (Recall the exposure
recommendations that accompany film advising for bright sun strong
shadows, hazy sun, no shadows, ect.) But when light is observed without
the benefit of experienced conditions, the absolute value of intensity
cannot be determined. This can be because of the ability of the eye to
see brightness from very dark to very bright. And not only that, but
able to recognize relative values even though at various intensities
(that is a gray card will look like a gray card relative to surroundings
whether in dark shadows or in bright light).
An interesting study is to view a photograph is dim and bright light.
The photograph will basically be recognized as identical in either
situation, even the contrast and color will appear correct. As the
lighting conditions change, the eye and mind tend to normalize (recall
how a sheet of white paper still seems white under tungsten lighting, it
only becomes a warmer red when we compare to how it looks illuminated by
the sun at noon.) More important is that the contrast will seem correct
(as printed) when viewed with either illumination unless reference is
made between the illuminations.
The light meter provides information as to the absolute amount of light
in the range of interest. This provides the basic exposure and contrast
of the scene. The film is then exposed and developed as desired
referenced to the absolute amount of light from the subject.
Now our needs change; we are no longer interested in an absolute value
of intensity but rather a relative value and relationship. The eye is
extremely accurate at relative determinations of intensity. It is the
relative intensity values that are important with negatives and prints.
These tell the contrast of the image and the relative darkness from
paper white (high key, normal, low key).
A full range print can appear to have a greater range than another
process even though the other process actually produces a greater
range. If one views together a full range Pt/Pd print with a full range
gelatin silver print, the gelatin silver print will have darker blacks
and lighter whites. However if the prints are viewed independently they
both appear full range from black to white. The Pt/Pd print may even
seem to have more range because of better tonal discrimination. Just
like looking at the photo earlier in dim and bright light, the eye veiws
relative amounts of light, not absolute.
The end product is a print which is viewed by the eye on a relative
basis. It is most critical that the proper relative values are placed
in the print. And recall also that values can and will be influenced by
neiboring values. Now if one wishes to print a density step calibrated
to absolute values, this can be a different matter, but to print an
image it is important to have the print values look as they should
relative to each other and the materials used (like the paper).
The densitometer measures absolute values (similar to a light meter);
the eye is superior at discriminating and judging relative values. What
is needed at the stage of evaluating a negative or a print are the
relative values, not the absolute values that were needed at the camera.
In real practice one may override the absolute measurements depending on
materials, technique, process, or desired results. What is important in
the final result are the relative placements of values.
> ... But, I intuitively know that i need help learning
> to read negatives for alt process! ...
I know I mentioned it before, but have you looked at Chapter 3 in my
guide. This proceedure is valid for any photographic printing process,
not just Pt/Pd. It lays out the variations in exposure and contrast in
the negative and the print so one can compair and learn the relative
differences for the actual materials used.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
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