I thought the list might enjoy this technical discussion of Tri-X and T-Max
in association with recent B&W threads. It was originally posted on the
Leica User Group mailing list.
- Reprinted with permission from the author Erwin Puts
Skip
X-Sender: imxputs@xxxxxxxxxxxxxx
Date: Mon, 3 Jan 2000 16:43:56 +0100
The current discussion on the relative or absolute erits ot Tri-X is
indeed refreshing. So a technical appraisal giving some background info
might be helpful. Classical emulsions like Tri-X use thicker emulsion
layers than Tgrain (modern films). The growth of silverhalide in the
classical emulsions is less controlled than in the tabular controlled
grain. Thus the shape of the individual grains in the Tri-X emulsion are
more irregular and the size of the grain has a wider span. You will find
very small sizes and very big ones and everything in between. So we have a
thicker emulsion and more irregularly shaped grain that is distributed
over a larger bandwidth of size. If we have larger and smaller grains
side by side we increase exposure latitude as the small grains are less
sensitive than the bigger ones. But the shape and size distribution has
one more effect: it enhances the internal (micro) contrast and promotes
the recording of subtle tonal differences. Again courtesy by the wider
latitude in distribution of grain size and shapes. If we have grain
clumps in all sizes the chances are quite good that small variations of
tonality in small subject areas can be captured more easily by this
assortment of grain sizes and shapes.
Another point: D76 is a halide solvent type of developer, that reduces the
grain clumps during development and gives them soft edges. This tendency
flattens the internal contrast and when used with modern (Tgrain) films
which inherently are of flatter micro contrast, the effect is doubled.
That is why the tonal rendition of Tmax in D76 is markedly different from
that of Tri-X in D76. Another often overlooked point is that
filmdevelopers function as chemical or physical development processes. The
difference is a difference in grain clumping, but also the silver deposit
may be black or sivery. Both types of course reduce the light
transmission, but the black silver gives a different quality of light than
the grey silver.
Of course if you are well versed in the intricacies of these effects and
the ways of influencing them/compensating them you may use any
film/developer combination and emulate quite effectively any other one.
But when you opt for one of two different strategies/materials you end up
with intriguing results, which are more predictable if you know the basics.
The tonal rendition is also strongly influenced by the shape and slope of
the characteristic curve. Any film has a density range of zero to 3.0 (in
log units). Any stop difference results in a value of 0.3. So
theoretically any film can accommodate 10 stops if you go for a 45 degree
contrast index, that is 1.0 . Now it is more natural to use a 0.7 value
and that results in the claim that a film can handle 15 stops. This is not
true of course. The effective range of a film is from 0.1 to 1.4, that is
a range of 1.3, This range can accommodate 7 stops when we use the CI
value of 0.7 and 9 stops if we use a CI value of 0.5. Lowering the CI
value is the same as underdeveloping the film. If we now use the seven
stop range as a standard, we see from the non-linear characteristic curve
that the rendition of tonal differences depends on two things. The
steepness of the straight portion and th selection of the portion of the
tonal scale of the subject you want to record faithfully. In the toe and
shoulder of the film curve things are compressed and so do not record
correctly. We may like this compression effect, mind you.
If we now have a Tri-X film with a characteristic curve that is sagging a
little in the midtone area we compress the midtones and flatten contrast,
making it look like a Tmax neg. On the other hand I can use a developer
that gives the curve an upward shape in the midtones and so enhance micro
contrast in the midtones. In this way I can use T-max film and give it a
look as a Tri-X film. You need suitable developer, careful exposure etc,
but it can be done. Not an exact emulation but close.
We should also be aware of the absolute tonal difference between two grey
values. If the differences in reality are small, we will make them smaller
still when using a CI value of 0.5. It may be that the differences become
too small to be perceptible. (re Fechner's law). So if we are going to
compare the tonal rendition of two film emulsions we should use identical
subject tonal ranges AND identical CI values. Otherwise any comparison
goes wrong. Same as with lenses that is.
Current Tri-X is not the same emulsion as it was in the past. There are a
number of changes, making it behave a bit more like Tmax. At least the
amount of silver is reduced and so presumably is the range of grain sizes.
Erwin
Please note: all my articles about technical matters, even when posted on
the Lug/Leg are copyright by me. Do not use without due references.
-----------------------------------------------
Skip Williams
Westfield, NJ
skipwilliams@xxxxxxxxx
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