Re: Developing Time for 2-stop Pull?


"UC" <uraniumcommittee@xxxxxxxxx> wrote in message
news:1174841196.485723.304790@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Mar 24, 6:50 pm, "Richard Knoppow"
<dickb...@xxxxxxxxxxxxx> wrote:
"UC" <uraniumcommit...@xxxxxxxxx> wrote in message

news:1174671446.459847.24270@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Remember I said EXTREME overexposure.

Its true that reducing development
time reduces contrast but overexposing does not decrease
contrast as long as the exposure is within the range of
the
film.

EXTREME overexpopsure will place a lot of the highlight
area on the
shoulder, where the slope of the curve is lower than
average. Thus,
reducing development will bring about an extremely flat
negative.


Agreed, but am not sure increasing development will
increase contrast here. The shoulder occurs because the film
has reached its maximum density, increasing development will
simply raise the lower densities to the same limiting value
decreasing contrast. In this case the exposure was two stops
over the ISO value, really not even out of the "normal"
range. It would take many stops of overexposure to get to
the point where compression due to shouldering took place.
Note that the current ISO speed method is still based on
the minimum exposure necessary to place the shadows on a
part of the toe of the curve with usable gradient. This is
based on Loyd A. Jones original concept. Jones conducted
extensive experiments to determine the _practical_ speeds of
materials in actual use. His idea of keeping exposure
minimal was because grain and sharpness are better for
generally thin negatives. This is not so much true now as it
was in the 1930's and 1940's, when most of this research was
done, but its still true to some degree. What Jones found
was that good tone reproduction depended on the shadows
falling on the toe of the film where the gradient (contrast)
was not less than 1/3rd of the contrast of the straight line
portion of the characteristic curve. Increasing exposure did
not affect tone reproduction over a range of many stops, but
less exposure resulted in poor blacks and overall lower
quality.
Jones' research was done by making many negatives of
several subjects with varying exposures and getting the best
possible print from each. These prints were viewed by a
large number of people as a series. They were asked to
choose those that were considered to be "excellent". The
print with the lowest exposure that was considered
"excellent" or first excellent print, in the language of the
research reports, was used to determine the minimum exposure
necessary. The resulting speed system was used internally by
Kodak for several years. Some books from the 1940's list
Kodak Speeds for their films. These are about four times the
current ISO speed.
The first speed system adopted by the American
Standards Association (now ANSI) was based on the Kodak
method but, for some reason, added a 2.5X fudge factor. This
resulted in overly dense negatiges, just what Jones wanted
to avoid. This speed method was used from about 1944 to
1958. At that time the ASA decided that the Kodak method was
too difficult to measure in practice, in addition, it was
realized that the fudge factor was excessive. In the
meantime the German standards organization, who publishe the
DIN standards, had come up with a new speed measuring sytem
based on a fixed minimum density above gross fog and base
density. The ASA conducted a long series of tests of many
films and determined that the fixed density bore a constant
relationship to the minimum gradient point of the toe. By
introducing a 1.25 multiplication factor the speeds derived
from the DIN method agreed with those obtained by the Jones
method. This new system was adoped by the ASA and DIN, it is
the method in use now although some small revisions of the
standard were made over time. So, the current ISO method
requires about the minimum exposure possible for good tone
rendition. However, again, increased exposure will also
result in good tone rendition over a range of many stops, as
found by Jones in his old research.
An important consideration of published film speed is
the developer used for the test. The original ASA method
included two developers, a standard developer and a "fine
grain" developer, neither of which was identical to any
commonly used developer, although the "fine grain" one was
close to D-23. Since developers do have an effect on speed
this often led to the box speed yeilding less than optimum
results. The current ISO standard no longer specifies a
"standard" developer but allows the use of any developer,
provided that it is specified along with the resulting
speed. The total range for developer variation, except for
some odd developers, is on the order of +/- 1-1/2 stops, so,
its important to know what sort of developer was used for
establishing the ISO speed.
Note that the ISO has many standards applying to various
kinds of film. The standard we are discussing applys _only_
to B&W silver negative film for still cameras. Motion
picture films, reversal films, color films, aerial films,
etc., all have separate standards. This is partly because
the processing methods are very different or because
sensitometric requirements are different. In the case of
color film and motion picture film the processing is much
more standardized than for B&W still negative film so the
speed standard reflects this. Note also that these film are
usually developed to a much more standardized contrast. B&W
negative films are developed to a broad range of contrast
values which affects the _effective_ speed. The ISO method
results in fairly high contrast, about right for contact
printing or diffusion enlarging. If the same film is
deveoped to a lower contrast, say for use in a condenser
enlarger, the speed is reduced and the ISO speed no longer
applies.


--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx






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