[rollei_list] Re: looks like ... agfa Sala

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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