[pure-silver] Re: Tonal gradation/smoothness in 35mm negs c.f. larger formats
- From: "Nicholas O. Lindan" <nolindan@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 14 Jan 2008 09:49:09 -0500
"Peter Badcock" <peter.badcock@xxxxxxxxx>
why I can't get the same tonal gradation/smoothness ... from 35mm
[Vs. medium/large format]
You can, easily - if you limit yourself to 1 x 1 1/2" contact prints.
If possible, assume grain size is small enough to be irrelevant
Then there is no discussion - the name of the game is grain.
Film is nothing but grain and something to hold the grain in place.
The issue is 3-fold:
1) Grain size
2) Grain density
3) Grain size distribution
1 - Grain size has to be small so a) you don't see the grain; b) can
fit a lot of grains into a small area.
2- Grain density has to be high. Grains are quasi-digital: you get a
black splodge or you don't. The percentage of grains that get hit
by light controls gradation. If you have one grain/mm^2 - the grain
can be any size - then there are only two tones possible: 0) base;
1) Dmax. To get 4 tones you need to have 2 grains/mm^2, 8 tones require
3 grains/mm^2 ... And it gets a lot worse, density is logarithmic and
number of grains is linear so highlights are well gradated but
shadow detail/gradation is low. To get 256 evenly spaces tones
in a unit area (a pixel) you need to have 2,700 grains
(dots/pix <- (dots/inch^2 / pix/inch^2)).
Microfilm is fine grain but thin emulsion (few grains/unit area).
As a result, even though grain is non-existent, and resolution
is high the gradation is lousy. (That grain size is uniform only
makes the situation worse.)
3 - Grains have to be of different sizes: if all the grains are the same
size then the film is high contrast. Once a grain has been activated
by light any more light falling on the grain does not increase the
film density. If all the grains are the same size then the probability
of grain being activated is uniform and once a certain intensity of
light is reached then suddenly all the grains are activated and there
is no further response and image is high contrast.
If grains are of differing sizes then the large grains have a higher
probability of being activated than smaller grains. All the large
grains may be activated but there are still plenty of small unactivated
grains left to build density.
TechPan was a thick ultra-fine grain emulsion with a wide range of
grain size.
The formulation for TechPan changed c. 1984/5 and the film lost its
large-format look (ref. "Controls in B&W", Dr. R. J. Henry & personal
experience).
And, so, Tmax-100 film with Microdol-X has a look close to contemporary
TechPan and is the highest image quality film/developer combination
available, where image quality is the combination of resolution and
gradation.
The difference between TMX/M-X and even modern TP/Technidol is clearly
visible
if you use a step tablet as a resolution target. Both films have about
the same resolving power for a white/black target. For a gradated
target TP shows distinct density patches with sharp demarcations
while TMX can produce only a smear.
==
Nicholas O. Lindan
Cleveland Engineering Design, LLC
Cleveland, Ohio 44121
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