Richard Knoppow <dickburk@[EMAIL PROTECTED]
> wrote:
>
> This is getting long but I can't find anything to snip.
I hope that means that it was mostly ok.
> I don't know when Kodak began using Jones' method
>internally but it did begin to publish Kodak speeds around
>1939. Jones actually worked out his system much earlier.
I'm sure that's right, but published Kodak speeds
from before 1939 are a different system. The Kodak
speeds listed in the 1938 edition of "Eastman
Professional Films" are not the Jones system.
The pre-1939 Kodak system was based on inertia
(Like Weston and H&D) and is equal to ten times
the Weston speed.
> The ASA adopted the system in 1943 with a
> safety factor of 2.
That's what you've said several times.
But - In "Kodak Films" fifth edition 1951
from the Kodak Reference Handbook on page 16,
it says "For the black-and-white continuous
tone negative materials covered by the standard,
a safety factor of 2.5 is used."
In "Kodak Films" seventh edition 1956, it says
the same thing on the top of page 21.
> ASA speeds were 1/4 of Kodak speeds. The resulting
>number could be used with either Weston or General Electric
>meters of the time with insignificant error. However, the
>safety factor increased the exposure by a stop over the
>Jones speed point. A film rated Kodak 400 would be an ASA
>100 film by this standard. In its data sheets of this time
>Kodak stated that the exposure could be reduced a stop if
>work was carried out carefully.
That's true, but I expect you have the same booklets
I have where it says the safety factor is 2.5.
> The second ASA standard changed the method of
> measurement from the Jones minimum usable gradient to a
> fixed minimum density method as adopted by the DIN in the
> early 1950's (don't have the exact date at hand).
The 1958 Ilford Manual by Horder has the date for
revised DIN standard as 1957 (p. 284, 287).
The older 1936 DIN system used the same minimum
density requirement, but specified development
for maximum speed, rather than a standard
development representing something like
normal use - as was used from 1957.
I think the ASA Standard is ANSI PH2.5-1960, at least
it is cited that way in Photographic Sensitometry
by Todd and Zakia (1974 p.164).
These would just be publication dates, the actual
work would always be a bit earlier.
> However, they wanted to accomplish two things:
> first was to make the speeds compatible with
> earlier ASA speeds,
That made a lot of sense given the number
of meters already in use.
> and secondly, to maintain something like the
> Jones idea of the minimum gradient.
It doesn't use the 0.3 times average gradient
criterion though, it uses the 0.1 density
above base + fog criterion.
> The ASA conducted extensive surveys of films of
> the time and found that there was a nearly constant
> ratio between the fixed minimum density, that is
> log 0.1 above fog plus sup****t density, and the
> Jones point as found using the Jones method.
It seems to hold good within a third of a stop
based on the ratings of films just before
and just after the change.
Note that the inertia based Weston speed also tracked
pretty well with the Jones Kodak Speed and Old ASA.
The normal relation****p would be that Weston 40
equaled Kodak 200 and Old ASA 50. Many films
seem to have fit that relation****p perfectly
and I'm unaware of any that were more than
1/3 stop off.
The key seems to be normal development.
Weston, Old ASA, and ISO (New ASA/New DIN)
use different criteria, but track quite well.
They all use normal development.
Old DIN (1936) tracked the other systems very badly.
It used development for maximum speed.
The same criterion, but with normal development,
works very well.
> This obviated the difficult Jones measurement.
> The ratio turned out to be about 1.25 times
> the exposure required to reach the DIN density point.
The math in my previous post showed that
an ISO 400/27 film has a DIN density
point at -2.7 log lux seconds and that
an Old ASA 200 film had a Jones Point
at -2.9. Unless someone shows that
my formulas or math are wrong, I'm
sticking to those figures.
The Jones Point for a typical film
is thus 0.2 log units to the left
and represents 2/3 of a stop less exposure
than the DIN point.
> So, a factor of 0.8, the reciprocal of 1.25
> is introduced into the calculation of the speed
> in the new ASA method.
That was to get the numbers they wanted,
but the actual difference between the DIN
point and the Jones point for films
where New ASA is double Old ASA is 0.2 log units,
2/3 of a stop or a factor of around 1.6.
> In effect the speeds were now double those measured
> by the old ASA method and half of the Kodak speed.
True for most films.
> I reiterate that the factor in the current speed method
>is NOT a safety factor but rather to bring measurements made
>by the method into agreement with the speed that would be
>measured by the Jones/Kodak method and reverts to Jones'
>original idea of finding the minimum exposure that results
>in good tone rendition.
Yes but if the "Kodak Films" booklets are correct
that Old ASA had a safety factor of 2.5, then the
doubling of the speed ratings should have reduced
this to 1.25. I agree that this has nothing to
do with the 0.8 in the formula. The 0.8 in the
formula just puts the scale where they wanted it.
> The original Kodak method does not seem to have had a
> fixed contrast, however, contrast does affect the speeds
> measured by either method. The Kodak method does specify a
> fixed exposure interval, much the same as the current
> method.
Yes, but changes in development time tend to affect
the contrast in the toe and the overall contrast
at the same time. Since the Jones method depends
on the ratio between the slope of the curve
at the Jones point and the overall slope, changes
in development will have less effect on the
Jones point than on the DIN point.
> Since the old ASA method and the new ISO method are
> compatible as to speed point even though measured by
> different techniques, its possible to translate old ASA (pre
> 1958) speeds to equivalent modern speeds by simply
> multiplying by 2. Keep in mind that the speeds in both
> systems are rounded off as were old Weston speeds so there
> may not be an exact agreement. Also, even though Kodak used
> the same trade names for decades the emulsions were changed
> many times. Current Plus-X is not the same as the product
> of, say, 1948 although its broad speed category is about the
> same and its intended use is about the same so speed
> comparisons must be made with some care.
Plus-X 35mm did vary in rated speed over the years.
Kodak 200 in 1943 (Old ASA 50)
ASA 50 in 1951
ASA 80 in 1956
But the speed of both Plus-X 35mm and Verichrome
Pan just before the change was ASA 80 and both
were ASA 125 just after the change and forever after.
I'm aware that both systems round to the nearest
1/3 stop so that a 1/3 stop difference in reality
may be hardly anything at all.
My impression is that Plus-X 35mm, while now
greatly improved from the film introduced in 1938,
never underwent any rapid obvious change from
year to year. Every now and then the new stuff
would be just a little better. A bunch of minor
changes over 69 years can add up a lot.
Then again, it was always around 100 speed
by today's standards. It was always a double
coated film, always type B panchromatic,
always had a high acutance, and was always
fine grained for its time.
> What is surprizing is how fast films of the mid 1940's
> were. The difference was, of course, grain. A 1940's film
> which would measure 400 on the ISO system would be extremely
> grainy compared to a modern film of that speed.
Medium speed films were actually pretty ok.
I have some of my father's Verichrome negatives
that he took with his Brownie in the 1940s.
Even with department store processing, the
negatives are still a bit finer grained
than today's Tri-X in D-76 1:1.
Peter.
--
pirwin@[EMAIL PROTECTED]
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