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

----- 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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