On Nov 1, 8:48 pm, Peter Irwin <pir...@[EMAIL PROTECTED]
> wrote:
> Richard Knoppow <dickb...@[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.
> --
> pir...@[EMAIL PROTECTED]
I have ever read says the safety factor before 1959 was
2.5. It was reduced to 1.25 in the new system. The right factor is
about 1.75.
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