FotoDave@aol.com
Wed, 14 Apr 1999 14:26:26 -0400 (EDT)
Well, when I first start the lith test, I thought about writing about flash
for fine control but never get the time to do it, but since it has been
discussed online, I will simply type it in briefly. Hopefully this will help
a little for those doing tests. Note that exactly same principles hold for
direct and reversal processing.
1. The density of your original negative determines your normal exposure.
This is not surprising, of course, as this in the principle in any process.
2. The density *range* of the original negative determines your
multiplication factor. The principle is that you exposure the shadow (shadows
of the image) to the shoulder region of the positive or the toe region of the
reversed negative.
You could go theoretical on this and find the factor densitometrically; but
the best image is not always that we match the clear part of the film to
Dmax. It is an aesthetic judement (zone placement). That is why I suggested
using test strip including multiplication factors instead of find the
"normal" and then magically apply some number.
3. Flashing can be used to enhance the separation of highlights (of the
image). It basically give some additional exposure to the toe region of the
positive so that highlights fall into the linear region. When reversed, the
highlight then falls into the linear region of the shoulder.
When more flashing is given, the highlights of the positive get fogged
(density raised); on the reversed negative, the highlight densities get
reduced. Thus you can use this to control the density range of your positive
or negative.
SENSITOMETRIC (STEP-WEDGE) TESTS
==============================
For those interested in formal step-wedge testing, I will outline the
procedure:
1. Determine some exposure time so that you cover *BOTH* the shoulder and toe
regions. As long as you cover both region, the multiplication factor does not
mean anything anymore since it basically shift the scale up or down.
2. Determine the "critical flash time." This is the maximum amount of flash
before the flash introduce any fog to the positive. I usually call this the
"maximum allowable flash," but that is for paper printing since if more flash
is given it will produce fog, which is bad in paper printing. However, in
film, the extra fog can be printed through in negative, and the extra fog
changes the density range of the reversed negative, so more flash than the
"maximum" can be used, thus "maximum" doesn't describe well. I will call it
the "critical flash time" or "critical flash exposure."
3. Expose different sheets according to time in #1, then flash each one with
different falsh time, starting with no flash, critical flash, and then
increase by half stop. For example, if you critical flash time is 4 seconds,
do a series with 0, 4, 5.6, 8, 11, 16, .... seconds.
Then you plot the curves, and it basically "define" the process. Once you see
the curves, you will know exactly what the normal exposure, multiplication
factor, and flashing are doing. Don't just use the curve. Think about why the
curve changes in a particular way. If you completely understand why it
happens, you understand everything about sensitometry.
You will find:
1. The no-flash film simply gives you the characteristic of the lith film at
your particular development. The shape is basically the reverse of the curves
which were shown in the journal, issue #2, on continous-tone development of
lith film.
2. The critically flashed film will show definite enhancement of highlight
separation, but the density range of the reversed negative is basically the
same as that from #1 above.
3. The more than critically flash films will lower the density range of the
reversed negatives. This is because the shadow stays more or less the same,
but the highlights are raised in the positive and so lowered in the reversed
negative.
4. Except for the enhanced highlight separation achieved with critical flash,
you cannot expand the exposure range of the lith film. That is, the number of
separable steps remains the same. Of course, one can argue that when the
contrast is reduced, more steps can be considered in the linear region. This
is true in application sense, but technically the exposure range remains the
same.
I haven't done any reversal processing on lith film (and so I will get
severely critisized for posting this), but for those who will do the test,
you will find the result the same or very similar to what I described above.
It is simply the character of the film and development; there is nothing
magical (or even new) about it. Theories help. And the theories match every
curve that I plot using the data that Liam has posted online or sent to me.
I hope that this not only helps shorten some testing time but also helps some
to see the beauty of sensitometry and hopefully that will interest you in
studying more about it in the future.
Sincerely,
Dave Soemarko
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