[rollei_list] Re: OT Infared film and shooting with it.
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Tue, 25 Sep 2007 22:24:05 -0700
----- Original Message -----
From: "Marc James Small" <marcsmall@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Tuesday, September 25, 2007 9:23 AM
Subject: [rollei_list] Re: OT Infared film and shooting with
it.
At 11:35 AM 9/25/2007, Eric Goldstein wrote:
>The manufacturer's rated speed of the film is its ISO...
there is no
>more ASA...
>
Har. Har. Har. "ISO" is just a combination of
DIN and ASA. That is, film was once listed as
either 21 DIN, for example, or as 100 ASA. Today,
it is listed as "100/21 ISO", but even the Nikon
users on the List can decipher this as "21 DIN"
or "100 ASA". It is NOT rocket science, so to speak.
So, yes, it remains proper to speak of ASA
speeds, as these remain part of the ISO system.
Marc
msmall@xxxxxxxxxxxx
Cha robh bàs fir gun ghràs fir!
I don't want to reiterate my rather long post about
this. ASA is the American Standards Association. This was
renamed the American National Standards Institute, probably
because people can't stand simple names. The ISO is an
international association of national standards groups and
attempts to co-ordinate standards. An ISO standard is also
an ANSI standard.
The DIN, which is the German association for
establishing industrial standards, is also a member of ISO.
The original film speed standard adopted by the ASA in
the United States, in 1943, was a modified version of a
method which had been developed at Kodak and used internally
there for several years. The method was devised by Loyd A.
Jones after many years of research on determining the
_practical_ exposure of film for pictorial use. Jones's
method required measuring intercepts of a double slope and
evidently was not very simple in practice. Jones idea was
that the minimum exposure point should be at a high enough
contrast (or gamma) point on the toe of the film curve to
provide for some shadow detail. His research found that tone
rendition of prints improved until the negative exposure
reached this point but that there was little detectable
change for greater exposure. He chose to determine minimum
exposure because film tends to become more grainy with
density and also to loose some sharpness.
Unfortunately, the original ASA version of this included
a two times safety factor so that negatives were inevitably
too dense thus destroying the purpose of the standard.
In 1958 the ASA reviewed the method for determining
speed. It examined the method used by the DIN at that time.
This was _not_ the original pre-WW-2 DIN method but one
which measured a speed point determined by a minimum density
above gross fog and support density. The ASA conducted tests
of a great many emulsions and found that a fixed multiplier
of this minimum density point would result in practical
aggreement with the speeds determined by the Jones Mimimum
Gradient method but was much easier to measure reliably. The
ASA adopted this system with the fixed factor (about 1.25)
and eliminated the two times safty factor previously applied
to the Jones speeds. The practical result was the doubling
of all published film speeds.
Film speed is still given in both ISO and DIN even
through the two are identical methods because ISO speeds are
stated in an arithmetical series and DIN speeds in a
logrithmetic method. This is to match existing light meter
calculators.
The ISO/DIN method specifies the exposure range and
density range to result from it, thus it specifies the
contrast or gamma of the negatives. The last change in the
standard eliminated the specification of a standard
developer, however the developer must be specified along
with the resulting speed.
If a film is developed using a different developer than
that used for the measurement, or is exposed and developed
to a different contrast index (or gamma) the ISO/DIN speed
no longer applies. However, the ISO/DIN speeds still give at
least a rough estimate of the differences among films.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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