[pure-silver] Re: hp5+ at 125 development times
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Tue, 6 Oct 2009 16:28:02 -0700
----- Original Message -----
From: "Shannon Stoney" <shannonstoney@xxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, October 05, 2009 11:34 AM
Subject: [pure-silver] Re: hp5+ at 125 development times
The subject was kids' portraits, done outside, in the
afternoon.
--shannon
I sent an incomplete message inadvertently.
Some notes on film speed and development.
Film speed as specified by the manufacturer is
determined by a very well controlled procedure specified in
an international standard. The ISO standard currently used
specifies an effective contrast index by giving a range of
exposure which is to produce a range of densities. Its done
this way to make it independent of the H&D curve of the
film. The standard specifies the method of exposure and
density measurement, requires that the exposed film be
stored for a time to compensate for possible latent image
instability and has other controls. In the past the standard
also specified a developer or even two developers (normal
and fine grain) but it no longer does. The tester can use
any developer _provided_ its specified along with the speed
ratings.
The contrast index which results from the conditions of
the test is about that normally required for printing with a
diffusion light source. When a lower contrast is required
the film must be exposed and processed differently and that
will have an effect on the speed. The ISO standard includes
a chart showing the effect of a change in these conditions
on contrast.
The speed given by the current ISO standard is such as
to yield a negative with the minimum exposure required for
good shadow detail. This criteria is inherited from work
done at Kodak Research Laboratories some sixty to seventy
years ago by Loyd A. Jones and others. Their reason for
finding the _minimum_ exposure is that both grain and
sharpness of the image vary with density being best for
relatively thin negatives. The research was carried out by
making hundreds of negatives of various typical scenes at
various exposures, making the best possible prints from
these negatives and having the prints judged in a
double-blind test by a large jury to find the "first
excellent print" i.e. the print from the thinnest negative
that yielded a good print.
From this and other data Jones, et.al. determined the speed
point should be such as to put the minimum density that was
to print with any detail (shadow detail) at a point on the
toe of the film curve where the contrast was about 1/3rd of
the overall straight line curve.
They also determined, that aside from grain and sharpness,
most films had a tremendous exposure latitude for increased
exposure, that is, negatives which received greater exposure
than the minimum had tone rendition which was equally as
good as the "first excellent print".
This system was used internally at Kodak for a few
years and some "Kodak Speeds" were published. In 1943 this
system was adopted with some unfortunate changes to be the
first ASA speed standard. The original ASA standard ignored
the whole idea of minimum exposure and added a one stop
safety factor. This resulted in general in overly dense
negatives. The idea was that amateur photographers wanted
some sort of picture and the latitude for over-exposure was
enormously greater than for under-exposure. Kodak notes in
some of its later literature that higher film speeds than
those specified can be used where the photographer is sure
that his/her controls are good.
The Kodak system worked well but determination of the
speed point was very difficult so this system was abandoned
when the ASA revised its standard in 1958. The new system
was based on the then new DIN system developed in Germany.
In the DIN system the speed point was determined by giving
the film a controlled exposure and development and finding
the exposure for a density of log 0.1 above gross fog and
support density. When the ASA adopted this basic system it
did considerable research and found that the point on the
toe part of the curve required by the Jones/Kodak system
would be the same for either system when the DIN exposure
was multiplied by a factor of 1.25. This basic system is
what the current ISO system essentially is. The ISO standard
has been revised several times over the decades its been in
use but the most important change was the last revision
which dropped the use of a standard developer. Because the
developers specified in the various versions of the standard
that had them were not similar to commercial developers it
was decided that it made more sense to allow the use of any
developer provided it was specified along with the results.
Developers can have an effect on speed but the overall
range of fastest to slowest is relatively small. D-76 has
been for many decades the "standard" by which other
developers is judged. Developers with low activity or large
amounts of halide solvent, for example Microdol-X and
Perceptol, both when used full strength, will lose about 3/4
stop of speed when compared to D-76. Some developers
containing Phenidone or its relatives, for example Microphen
or T-Max, will deliver about 3/4 stop more speed than D-76.
About the only developers which are out of this range are
the long obsolete PPD developer of the 1930s which can lose
many stops of speed.
Note that tone rendition is not affected between the
point of minimum needed exposure and exposure many stops
greater, the range depending on the film. Modern films do
not "block up" that is have very low contrast in the
highlights, unless given tremendous overexposure or if used
with a developer which is unable to produce the highest
densities of the negative. Some dilutions of HC-110 and
similar developer do tend to produce a "shoulder" on the
film curve but even these will yield good negatives from
film which has been given substantial overexposure.
Many photographers find that the speed given by the ISO
standard does not give them acceptable negatives so increase
exposure by about one stop.
Of course, if you want negatives with a lower or high
contrast than that used in the ISO measurement the
_effective_ speed will be changed. The change in exposure
index is about 3/4 stop for a one paper grade change of
negative contrast.
I have used the term Contrast Index here. That is a
Kodak term which refers to contrast as measured from a low
to high density point on the film curve without regard to
the shape of the curve. The older term "gamma" is specified
to be measured at a point on the straight line portion of
the characteristic but some films do not have a straight
portion (Tri-X ISO-320 film for example) so gamma becomes
somewhat uncertain and Contrast Index is preferred. Average
Contrast is another method of measuring contrast but does
not specify the end points as the Kodak method does.
Your overexposed film is well within the range of
overexposure tolerance of even older film.
The effect of increased density on grain and sharpness
of modern films is considerably less than on the films used
by Jones, et. al. so somewhat denser negatives are quite
acceptable and will give you good tone rendition.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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