[pure-silver] Re: Pre-soaking film
- From: Eric Nelson <emanmb@xxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Fri, 30 Apr 2010 22:38:24 -0700 (PDT)
Thanks, Richard, I had always wondered about that!
________________________________
From: Richard Knoppow <dickburk@xxxxxxxxxxxxx>
To: pure-silver@xxxxxxxxxxxxx
Sent: Fri, April 30, 2010 10:26:19 AM
Subject: [pure-silver] Re: Pre-soaking film
Roll film has to be quite flexible because in many cameras it must make
fairly sharp turns. 35mm film OTOH, usually follows a fairly straight line from
supply to take up spool. Sheet film is the thickest because it has to stay flat
in film holders. Now, film pack film was on the same support as roll film
because it had to be flexible to work in the film pack where it is pulled
around to the back when changing films.
35mm film was designed originally for use in motion picture cameras and
projectors where it is subject to considerable strain so must be strong and
resistant to tearing.
The support thicknesses have not changed much in many decades but emulsion
thickness has. One reason for the much improved resolution of modern films is
the reduction of "irradiation" or scattering of light within the emulsion
mainly because the emulsions are thinner. Since the emulsion is a fraction of
the thickness of the total film thicknes this does not make much difference to
the overall thickness.
Film flatness is determined by several factors, not least of which is the
camera design. In both roll film and 35mm still cameras the film is located
mostly by the back plate. Right after the film is wound it may be held taught
between the guide rollers but soon it relaxes so that the film or backing paper
of roll film lies against the back plate. Although back plates are often called
pressure plates they exert no pressure on the film. Rather they create a narrow
channel between the plate and guide rails that the film runs though. In
Rolleiflex and Rolleicord cameras the back plate is located by little tabs, or
what I call feet, that rest on machined reference surfaces next to the guide
rails. The shifting plate in later cameras serves to adjust the thickness of
this channel for either 35mm film or 120 roll film with paper backing. An
examination of the back plate will find that some have two sets of "feet" of
different lengths which are brought to
bear on a common reference surface pad or that there is a single set of feet
and two reference pads of different hights. The shifting of the back plate also
moves it out of the way of part of the Rolleikin but the main purpose is the
channel thickness adjustment. 220 film, because it has no backing paper should
require a narrower channel to properly locate the film although the difference
may not be enough to affect focus significantly.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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