[rollei_list] Re: looks like ... agfa Sala
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 30 May 2005 16:26:04 -0700
----- Original Message -----
From: "Carlos Manuel Freaza" <cmfreaza@xxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, May 30, 2005 3:57 PM
Subject: [rollei_list] Re: looks like ... agfa Sala
> Richard, I used Agfa Scala, I have the slides and I
> am processing Agfa 120 films regularly; to avoid a
> language/technical confusion, which is the base you
> are talking about?.
>
> All the best
> Carlos
>
I am asking specifically about the support of Agfa _35mm
negative_ films. 120 film and sheet films are usually
perfectly clear. Most 35mm B&W negative films have a gray
pigment in the support itself to prevent it from conducting
light lengthwise. This acts as a neutral density filter when
the film is made into a positive and projected. Scala also
has a perfectly clear support.
Because of the cross-section in the Agfa data sheet I
think its possible that the support of their 35mm
_negative_ films are also clear. I could settle this by
buying a cassette of APX-100 and trying it but someone might
have the answer.
A small clarification in terms. The word "base", short
for film base, and the word support mean the same thing,
namely the material the emulsion is coated on. In
sensitometry one sees the term "base plus fog" to give the
minimum density of the material. This gets misunderstood as
"base fog", which is a confounding of two things into one.
The base density is the density of the support alone. For
larger films is virtually perfectly transparent. Fog means
the minimum density of the emulsion with no exposure but
after processing. A certain amount of the silver halide in
the emulsion is developable even without exposure to light
for various reasons. Manufacturers try to minimize this fog
but some remains. Generally, the fog level is higher as the
film speed increases. Fog also can increase as the film
ages. When film speed is measured the reference density is
specified as being log density 0.1 above gross fog plus the
base density, whatever that is. While base density of larger
films is virtually zero, the density of most 35mm negative
films is fairly high, around logD 0.2, it must be subtracted
from the densitometer reading when measuring the density of
the _image_. The base density of 35mm film simply adds to
all other densities. In effect, its a neutral density
filter. For negatives the only effect is to increase
printing time a little. When film is reversed to a positive
the base density reduces illumination a little. If there is
nothing to compare it to no one will notice.
In general pigmented supports are not used for color
films. Most color film uses a different method of preventing
reflections from the support back into the emulsion than B&W
negative films. Most films of all types have a back coating
of gelatin to compensate for curling and to protect the back
from abrasion. In most B&W films a dye is added to this back
coating to absorb any light getting through the emulsion and
prevent it from reflecting back into the emulsion causing
halation. The dye is either removed or converted to a
colorless form by the sulfite in the developer and fixing
bath. Color films use a different system. They have a dense
coating right under the emulsion. This is effective both as
an anti-halation coating and to prevent light conducted
lengthwise through the support from reaching the emulsion.
This is important for 35mm still film and motion picture
films, where one end of the film is often exposed to bright
light. Without the anti-light-piping pigment or the under
the emulsion coating the light can be conducted by the base
into the part of the film in the camera or in the cassette
causing some of it to be fogged. The pigment in the support
is not removed by processing. In fact, it can't be removed
any more than the color of any colored plastic can be
removed by washing.
I hope this clarifies things (and doesn't make them more
confusing).
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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