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

Thank you very much for the detailed explanation,
Richard, it was necessary for me, in general we
associate "film base" with the film back (acetate
etc.) in Spanish.
I am only using color films for 35 mm, however if
nobody answers the question, I'll ask to a friend of
mine tomorrow.-

All the best
Carlos  




--- Richard Knoppow <dickburk@xxxxxxxxxxxxx> escribió:

> 
> ----- 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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