Liam Lawless (lawless@vignette.freeserve.co.uk)
Sat, 06 Mar 1999 19:58:45 +0000
Joao, Dave and anyone else who's been following,
In offlist correspondence, Dave has pointed out that advice in my last
message to Joao is wrong as I have confused negatives and positives.
Guilty - I've been thinking back-to-front so much I've tied my brain in a
knot, so please accept this abject, grovelling apology, and disregard that
posting.
What Dave seems to be saying in his post of 5/3/99 is that the original
camera neg needs a short range in order to fit the straight-line portion of
APH's curve, and suggests 7-8 steps (not stops - this means 7-8 Stouffer
steps, which would equate to 3.5-4 stops, right?) A "good" camera neg with
a range of around 0.1-1.6 covers 6 stops (12 steps) and is too much for full
detail at both ends of the scale on lith, so I guess that Dave's figures are
about right.
However, we do not always expose and develop our camera films with the
express intention of making enlarged negs for platinum, gum or whatever (at
least, I do not, as I also like to print on bromide), so I am more concerned
with how to get the maximum detail from "normal" originals that have a
longer scale than reversal lith can handle, and spent last night pondering
this.
Regrettably, my experiments were not very successful, but I can offer some
advice on how best to deal with the type of negative I've described. I also
have another idea, described in the separate posting "Help wanted" which
probably sounds insane and which I cannot myself test at the moment, but
might be some use.
The important control for the contrast and density range is the base
exposure, or what exposure factor is applied to the normal exposure time.
When printing onto bromide, contrasty grades give the best shadow separation
but are not good at bringing out detail in the highlights, whereas soft
grades make the most of highlight detail but also give a low D-max and poor
shadow separation; where the negative range is excessive, a compromise must
be struck if split-grade printing cannot be resorted to. Similar
considerations apply to lith reversal, except that the situation is
reversed: we now want high contrast (equals a high exposure factor) for good
highlight detail, and low contrast (low factor) for good shadows.
Unfortunately, there does not appear to be an easy way to have both at the
same time (as with variable contrast papers).
To illustrate the difference the exposure factor can make, consider the
following two reversal negs, both assuming an original with maximum density
of 1.55 and aiming for a maximum density of about 2.3. (Figures from tests
I have carried out.)
Original (Stouffer) 0.05 0.20 0.35 0.50 0.65 0.80 0.95 1.10
1.25 1.40 1.55
3X/4 sec. flash 0.16 0.21 0.31 0.52 0.79 1.14 1.47 1.78
2.04 2.18 2.30
6X/3 sec. flash 0.14 0.14 0.17 0.20 0.28 0.48 0.77 1.20
1.61 1.96 2.28
3X gives what I would consider a good distribution, though, compared with
the original, the spacing (separation) is wider in the middle of the range
and slightly compressed at the ends (particularly the shadow end). With the
same exposure factors and shorter flash times, the difference between steps
increases proportionately across the range, but so of course does the
maximum density (an effect which would not always be wanted). If the flash
is increased so as to lower the maximum density, separation is reduced
across the range, again more or less proportionately. The general pattern
for any base/flash combination, in fact, is for separation to be greatest in
the midtones, and least in either shadows or highlights (depending on chosen
factor).
Extending the development time does not really do anything useful - which is
where I confused myself the other day. Simply increasing the time from 5 to
10 minutes gives a much lighter final negative (with, as I should have said
the other day, poorer separation, especially in the shadows). With the
exposure factor reduced to 2X from 3X and the flash time halved to maintain
a similar density range with the longer development time, there is in fact a
small increase in shadow separation, but at the expense of the extreme
highlights.
In conclusion, I have not managed to improve detail at both ends
simultaneously, though either end can be favoured by an appropriate choice
of exposure factor (low/high for shadows/highlights, respectively). Yet I
do not believe this to be a serious shortcoming of the method because, as I
have said before, a 3X factor produces something that works well for me (or
a 6X factor with pyro-stained originals).
What do others think?
Liam
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