[rollei_list] Re: B&W film developing
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 12 May 2014 11:16:56 -0700
----- Original Message -----
From: "Thor Legvold" <tlegvold@xxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>;
<rolleiusers@xxxxxxxxxxxxxxx>
Sent: Sunday, May 11, 2014 3:00 PM
Subject: [rollei_list] B&W film developing
Quick question for the people here on the list(s).
I’m working on getting my exposure and developmen dialed in,
and have read some conflicting info.
"Expose for shadows, develop for highlights" seems to be the
mantra.
This has already been discussed pretty fully but I will
add something. The characteristics of film were investigated
very thoroughly about eighty years ago by Loyd A. Jones of
Kodak Labs. He wrote several papers most of which were
published in the _Journal of the Franklin Institute_ at the
time a well respected peer reviewed journal of the general
sciences. Jones and his associates continued to publish
research as late as the 1950s. His goal was to discover how
film and paper render the tones of the original scene. Jones
worked out a method of determining film speed. Speed is
simply a way of being able to calculate the correct exposure
when the brightness range of the scene is known and the
transmission of light through the camera optical system is
known. Jones investigated both the effects of varying the
exposure and varying the development. What he found was that
for a given development, that is when the film was processed
to a specified mid-density gamma, and printed, there was a
minimum exposure that would result in an "excellent" print.
When the exposure was less the print quality was judged
inferior, but once that minimum exposure was given a further
increase in exposure had little effect on print quality over
a very wide range, many stops. Jones chose a speed point
near the minimum because the apparent grain was least and
sharpness greatest with minimum densities. Jones speed
point was such as to put the densities representing the
minimum shadow that was to have any detail on the toe of the
film curve where the gamma was about one third of the
"straight line" gamma. Kodak used this system internally as
"Kodak Speeds" for some time. It was adopted with some
changes, by the ASA about 1943. One problem was that the
ASA decided that since under exposure was a greater problem
than overexposure they would add a one stop safety factor so
published ASA speeds were about half the speed measured
using Jones method. Another problem was that measuring speed
by Jones method was difficult because it was based on the
gamma at two points on the film curve. About the mid 1950s
these problems led to a change by the ASA to the second DIN
system along with a couple of changes to it. The DIN system
measured a fixed density point. After considerable research
the ASA determined that when this speed was multiplied by a
small factor the result was nearly always identical to the
Jones speed but much easier to measure. The ASA adopted
this new method plus they dropped the safety factor. The
result is that all film speeds doubled overnight and a lot
of "magic" developers that guaranteed to increase film speed
disappeared.
The point of all this is that precise exposure is not
important for negative film provided its not to little.
Kodak adopted a system that is in some ways opposite of
the Zone system. What Kodak recommended was to expose to get
the information on the film and develop for a desired gamma
or contrast index, then adjust the contrast of the image by
choice of paper grade. Adams and Minor White worked out the
Zone system to insure adequate exposure after experiences
with badly exposed negatives that proved difficult to print.
I doubt if they were aware of the work of Jones, et.al. at
Kodak. One can work either way but there are some caveats:
one has to with the way the eye perceives images; we expect
a certain contrast in the mid-tones and tend to judge
overall images based on that. If you make an image low in
contrast in order to accommodate a wide brightness range in
the original image it may look flat and unnatural, the same
with increasing the contrast of a low-contrast image. I
think one has to try to get the mid-grays right and then, if
the highlights do not reproduce correctly use traditional
methods of burning and dodging, either by hand or with a
mask, to get them right. The real limit is in the range of
any reflection print that is viewed by ambient light. At the
widest, with glossy paper, this range is very substantially
less than can be recorded in a negative and generally less
than that in most original scenes. There really is no cure
for this other than manipulating the densities on the print
during printing to give the illusion of a full range of
brightness. Transparencies do not suffer so much from this
because they are generally illuminated in a way that can
produce highlight brightness well in excess of ambient
lighting.
Some additional notes: effective film speed is dependent
on development. The speed varies with the gamma to which the
film is developed and to some degree with the chemical
nature of the developer. The total range of variation with
different types of standard developer are about one stop
either way from a reference developer like D-76. The ISO
standard includes a chart to calculate the difference in
film speed when the degree of development is varied. The
standard specifies a brightness range for exposure and a
resulting density range. It does not specify a gamma
directly but does so indirectly through the above
specification. One reason for this is that many films do not
have linear density vs; exposure curves. The effective
contrast produced by the ISO speed method is about the right
contrast for contact printing or enlarging with a diffuse
light source. Printing using a condenser source may require
either lower contrast negatives or lower contrast paper. I
am not sure what effect, if any this has for scanning.
Of course, the Zone system also takes into account the
change in effective film speed with variations in
development to affect contrast.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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