[rollei_list] Re: looks like ... agfa Sala
- From: Eric Goldstein <egoldste@xxxxxxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Mon, 30 May 2005 17:26:34 -0500 (GMT-05:00)
The special knowledge is that Scala was on a transparent base; to my knowledge
exceptional in this regard and so uniquely suited to reverse processing.
I caught them calling the film ortho as well... I am assuming mistakenly but an
odd mistake given there are virtually no ortho films left...
Eric Goldstein
-----Original Message-----
From: Richard Knoppow <dickburk@xxxxxxxxxxxxx>
Sent: May 30, 2005 5:00 PM
To: rollei_list@xxxxxxxxxxxxx
Subject: [rollei_list] Re: looks like ... agfa Sala
----- Original Message -----
From: "Eric Goldstein" <egoldste@xxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, May 30, 2005 12:04 PM
Subject: [rollei_list] Re: looks like ... agfa Sala
> This will be helpful to your Richard:
>
> http://www.dr5.com/faqprint.html
>
>
> Eric Goldstein
>
Well, it starts out with a mistatement, namely that Scala
is an orthochromatic film. Its panchromatic. Also, by
"ordinary" B&W film they mean not chromogenic. The sentence
goes on to say its designed for reversal processing. We
already know that. I doubt if dr-5 has any special knowledge
about the film.
Reversal processing is not difficult but the first
developer must be tailored to the particular film. This
takes some experimentation. Reversal films are very critical
of exposure because the "negative" is developed virtually to
completion and the brightness range of the original subject
_must_ fit into the range of densities that will wind up on
the positive. The negative is developed to a gamma of about
1.0, this is fixed by the requirements of projection which
requires an overall gamma of 1.0. The other critical item is
the halide solvent in the first developer. The solvent is
necessary to get clear highlights in the reversed positive
and to maintain film speed. All films have a small residue
of very fine grain and very insensitive silver grains which
are never made developable by the original exposure. These
remain as undeveloped halide in the emulsion after the first
development. The bleaching step in reversal B&W _removes_
the silver but not the halide. That's why a dichromate
bleach is used instead of a ferricyanide blach. The latter
works by converting silver back to halide which is then
removed by hypo. We want the halide to remain in the
reversal film because it forms the final image after second
development. The highlight areas of the orginal negative
film are the most dense. After the bleaching step they
should have the lowest density right down to to the clear
base. When the very fine halide particles remain the are
made developable by the fogging of the film, either by
exposure to very bright light by a chemical, they are then
developed into silver, causing a veiling of the highlights.
Even very substantial increase of exposure of the orginal
will not cause most of these particles to become
developable. By adding a solvent to the first developer they
are removed during this step. Since the amount and nature of
the particles varies with the particular emulsion, and since
development time also varies, the amount of solvent must be
found experimentally. When it is right the film speed as a
reversal film will be nearly its speed as a negative and the
highlights of the reversed image will be clear. The most
common solvent in the older formulas is Sodium thiocyanate,
however sodium thiosulfate (hypo) is sometimes also used.
Thiocyanate is more effective and also has less tendency to
cause fog. First developers also usually contain a
relatively large amount of bromide to prevent developer fog
which will also cause veiling and general loss of contrast.
Because the first developer must produce rather high
contrast and maximum densities, and because of the large
amount of bromide and presense of the solvent, first
developers are generally very active. D-19 is a good basis
for a reversal developer. In the old days of soft emulsions
an auxilliary hardener was often recommended because of the
softening effect of the rather high pH first developer.
Second developers are not critical other than being quite
active and low in solvent power. A typical second developer
is a good print developer. Actually D-19, without the
solvent added can also be used as a second developer. When a
light fogging step is used rather than a chemical foggant a
final fixing step is required since there will be some
silver halide which will remain undeveloped despite the
initial solvent and everything else. Fogging redevelopers
like Sodium sulfide or conventional developers with the
addition of a fogging agent eliminate the need for the
fixing step.
Reversal processing was very common for 8mm and 16mm home
movies. Both the materials and processes were brought to a
high degree of perfection but whatever there is in the
technical literature seems to be rather obscure.
BTW, practical reversal processing is generally
attributed to John G. Capstaff, of Kodak Research
Laboratories. Capstaff was instrumental in devising the
entire system of 16mm motion pictures. Capstaff also
formulated D-76.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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