From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 12/28/01-04:37:39 PM Z
At 10:05 PM 12/27/2001 -0400, you wrote:
>DEAR JOHN,
> I have never seen a conventional film produce an image without
>development. There is no such thing as a sensitivity "built into a film
>{only} at its manufacture". ALL aspects of a film are related to a
>film/developer combination. Yes, of course, some films are faster, finer
>grain, etc due to manufacturing but all of those properties are manifested
>by film/developer/developing condition combinations...not the film alone.
>Manufacturing creates tendencies, development determines the extent to which
>they are manifested...very much like the ongoing debate about the effects of
>genetics and environment on personality traits and abilities.
> I am not talking about the ANSI standard speed which results from some
>very specific exposure and processing conditions (which hardly ever resemble
>the practical conditions of use), I am talking about practical usable speed
>for each users conditions.
>And Diafine or the newer version (I forget the name) for T-max films does
>yield a higher "practical" speed/sensitivity. The first thing I learned at
>RIT in 1969 was NOT to believe any manufacturers but to test under our own
>working conditions...and test we did until I had sensi-strips and practical
>shooting tests on the brain. It was and still is amazing just how far off
>most manufacturers claims are...but of course they can claim that it is due
>to "my" working conditions. "Not all that glistens is gold..."
> CHEERS!
> BOB
I must disagree with a couple of points.
The speed and grain of a film is pretty much determined by its emulsion.
Various developers will vary both to some extent but not a lot. Variation
of speed is perhaps plus or minus one stop due to developer
characteristics. In general, developers with Phenidone tend to increase
speed a little, those with lots of bromide or silver solvents tend to
decrease it.
Grain is also mostly a feature of the emulsion. Developers with high pH
and little sulfite tend to cause grain clumping due to migration of the
developing silver crystals toward each other. Large amounts of sulfite or
other halide solvent change the way the developer accesses the development
centers of the exposed halide particles and affect the shape of the
resulting silver crystals.
For the most part the shape of the characeristic curve is unaffeced by
development other than for shouldering off due to local exhaustion effects.
The ISO method of speed measurement is based on the German DIN method. It
measures speed at a minimum density. The contrast is fixed; a range of log
exposure and range of resulting log density is specified. This range is
based on the avarage scene brightness found by measuring many atual scenes,
the contrast is about right for contact printing and diffusion enlarging.
The current ISO standard does not specify a developer, as previous
standards did. The manufactuer can use any developer but it must be
specified with the speeds.
Changing the amount of development changes the contrast and results in a
different speed point since the minimum density is changed. For the reduced
contrast required for condenser enlarging the resulting speed change is a
loss of about 3/4 stop.
The purpose of the standard is to give a reproducible method of measuring
a fundamental emulsion characteristic so that they can be compared. Because
development of B&W materials is far from standardised the _effective_ speed
may vary from the ISO speed. This is not the case for color materials where
processing conditions are highly standardised. The ISO standard for B&W
negative materials does not apply to color materials, or even to B&W
materials use for other than still pictorial purposes (another standard is
used for B&W motion picture negative material for instance).
Differences between manufacturers specified speeds and those measured by
the photgrapher may stem from several causes. One is the use of some
developer other than the one used for the standard measurement, the other
is simply experimental error.
Because there is virtually no safety factor in ISO speeds there is very
little latitude for under exposure. Most films have a tremendous tollerance
for overexposure, as much as twelve stops for some. So, shooting film at
about half the rated speed may result in better tonal rendition than the
ISO speed.
A very great deal of research into speed rating methods and tonal
rendition was done by Lloyd A. Jones of Kodak Labs. He worked and published
from the mid 1920's until the late 1950's. The original ASA speed method,
adopted in 1943, was based on his "Kodak Speed" method. This system took
into account the shape of the toe region by specifying a point of minimum
usable gradient rather than a minimum density. The minimum gradient was
defined as being 1/3rd of the straight line gamma. For long toe films it
put the density up higher than for short toe film. Unfortunately, the ASA
decided to include a 2.5 X safety factor in the speeds. This resulted in
excessive density. Kodak's film booklets used to recommend halving the
exposure.+
The system proved difficult to use in practice, so around 1958 the ASA
changed its standard to the German DIN system, which used a minimum density
(the same log 0.1 over base and fog used now). At the same time the safety
factor was reduced to 1.5X, effectivly doubling the speed of all films on
the market.
The current ISO system is essentially identical to the 1958 system except
it no longer specifies a developer formula. The original standard and the
revised ones up to the current revision specified a developer, or for some
versions, two developers. The last one was essentially Kodak D-23.
>----- Original Message -----
>From: <Grafist@aol.com>
>To: <alt-photo-process-l@usask.ca>
>Sent: Wednesday, December 19, 2001 4:35 PM
>Subject: Re. Increasing Film sensitivity
>
>
>> Re the recent question on the possibility of increasing film sensitivity
>> after exposure.........After some reflection I would submit that the
>> "sensitivity" of a film to light is built into its emulsion at
>manufacture.
>> After exposure the sensitivity of the film would have been all used up, as
>it
>> were. The chemical structure of the silver halides has been changed by the
>> action of light thus creating a "latent" image.Then, all we could do is to
>> develop the latent but invisible image as fully as possible. See previous
>> postings. Alternatively, another possibility would be to treat the film
>> BEFORE exposure to increase its sensitivity. But that was not the
>question,
>> was it? What we are really thinking about is changing the manufacturers
>> rating of a film, innit?
>> Gordon wrote.......
>> >This is the practice of treating film after exposure, but prior to
>> development to >increase a film sensitivity.
>> See you. John - Photographist
>
>
---- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com
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