[LargeFormat] B&W film of the 40s

[Richard Knoppow]

----- Original Message ----- 
From: "Les Newcomer" <LNPhoto@twmi.rr.com>
To: <largeformat@f32.net>
Sent: Wednesday, May 28, 2003 5:15 PM
Subject: Re: [LargeFormat] B&W film of the 40s


> a friend of mine found his Kodak pamphlet of '41 and
another from '45.
> It corroborates Richard's assessment that Tri-x was around
in sheets,
> but not packs.   It also says the ASA speed was 200 not
800.  During
> this time Kodak gave their speed testing a 1 stop margin
of error.  In
> the 50s  they rescinded the fudge factor and all Kodak
films gained a
> stop in speed.
>
> so it comes down to
>
> Tri-x in sheets
>
> Plus-x or Super XX or Super Panchro in packs.
>
> Thanks to all.
> On Wednesday, May 28, 2003, at 06:24  PM, Richard Knoppow
wrote:
>
  It wasn't only Kodak. The 2.5X safety factor was built
into the ASA speed standard. Before the adoption of this
standard Kodak listed "Kodak Speeds" from about 1940. The
Kodak Speed system was the same method as used adopted by
the ASA but without the safety factor and without scaling to
approximately match the existing Weston and General Electric
meters. Kodak speeds are four times the ASA speed. Without
the safety factor they would be about twice the current ISO
speeds.
  Kodak speeds were measured by the method worked out by
Lloyd Jones, of Kodak Labs. This method took into account
the gamma of the toe section of the film curve. Jones
criteria for the minimum speed point was where its contrast
was 1/3 of the straight line section. He called this minimum
usable gradient. The system was adopted by the ASA in 1943
but with the added safety factor. The ASA dropped both the
minimum gradient method and the safety factor about 1958.
The minimum gradient method was too hard to measure in
practice and the safety factor proved to be excessive
resulting in overly dense negatives. The new ASA system,
which is essentially what we have now, was the German DIN
standard which used a fixed minimum density above base
density and fog. According to the ASA at the time the
measured speeds of many materials were made by both methods
and compared. They found no significant difference between
the two systems. So, the minimum usable gradient system was
dropped in favor of the fixed minimum density system with a
specified contrast index, or rather, a fixed range of
density for a fixed range of exposure. Details of the 1958
standard have been changed several times but the basic
method remains the same.
  As a result of dropping the safety factor the published
speeds of all B&W negative still films was doubled. Note
that other methods of measurement are used for motion
picture films, for color films, and for special purpose
films like aerial films. There are several standards in
current use.
  The original ASA speeds could be used on Weston meters by
using the next lower speed, on G.E. meters by using the next
higher. That is, ASA 100 would be Weston 80 and G.E. 125. If
you use _current_ ISO speeds on an old Weston meter set it
at half the current ISO speed. It will then agree with
modern meters.
  In their film booklets of the 1940's Kodak noted the
safety factor and stated that films could be used at twice
the ASA speed if care was used in exposing it.
  The safety factor actually went against Jones' intentions.
He set out to find the minimum exposure which would result
in an "excellent print". The idea was that thinner negatives
tend to be finer grained and sharper than denser negatives
although the tone rendition doesn't change with increasing
exposure beyond the minimum necessary for good shadow
detail, over a considerable range of stops.
  The safety factor may have originated with Kodak, who was
concerned with insuring that _something_ got on the film.
Since overexposure was well tollerated they evidently
decided to avoid problems with underexpoure, old film, old
developer, etc., by adding a one stop safety factor.
  The curent ISO method actually has about a 0.25 stop
safety factor.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com