[Leica] Tri-X technical look 2

[Erwin Puts <imxputs@knoware.nl>]

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

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