"UC" <uraniumcommittee@[EMAIL PROTECTED]
> wrote in message
news:1194009888.016672.32890@[EMAIL PROTECTED]
> 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]
>
> Everything 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.
>
I don't know what you mean by the "right factor" the
numbers 1.75 do not exist in the standard.
Also, the 0.8 or 1.25 factor in the new standard is
_NOT_ a safety factor as I explained in my long previous
post. It is meant to adjust the value gotten from the
straight DIN method to the speed which would result from the
Jones/Kodak minimum usable gradient method.
The ASA adoption of the Jones/Kodak method did include a
safety factor of 2.5. This was done to insure that there
would be a developable image when applied by amateurs.
Unfortunately it resulted in quite dense negatives. Jones
worked on the basis of determing the minimum practical
exposure for good tone rendition in order to obtain the best
sharpness and grain characteristic from a film. He also
found that increased exposure made little difference to the
tone rendition but that underexposure by even a small amount
resulted in poor rendition, so, the ASA and Kodak decided to
lower the film speed by about a stop to make sure people
would get acceptable results. Tone rendition was considered
more im****tant than optimum grain and sharpness.
Again speeds _re****ted_ by the current method are about
double those obtained by the original ASA method and about
half of those obtained by the method used by Kodak
internally. the division of the Kodak speed by a factor of
two fits the resulting speed to the calibration of the
exposure meter calculators current in the United States in
the 1940's, i.e. Weston and General Electric. Weston had
their own system of determining speed and all measurements
were made, at least at first, by Weston, so there was no
temptation for manufacturers to cheat, as they could with
the earlier DIN, Schneiner, or H&D systems, all being used
at the time. I have never found a description of the method
used by General Electric. GE speeds were two numbers higher
than Weston speeds and the original ASA sytem was designed
to result in a number in-between these two so it could be
used on either meter with insignificant error. Weston speeds
were rounded off so that all films within a bracketed range
had the same speed value. Probably GE did the same.
Actually, the current ISO standard also has rounded off
ranges so published speeds in any of these four systems are,
and have always been, approximations.
Again, the speed depends on the degree of development.
The ISO and new DIN standard effectively specify a contrast
index by specifying the density range to be obtained from an
exposure range. Any change in the contrast will affect the
effective speed. That's why the term EI, or Exposure Index
should be used when a speed has not been determined by the
ISO method.
Also, common developers can affect speed. The range is
around 1.5 stops overall for developers ranging from
Microdol-X or Perceptol (low end of speed when used full
strength) to Xtol, T-Max, Microphen (all at the high end of
speed) with developers like D-76 being in the middle. The
standard requires that the developer used be specified with
the speed ratings. Kodak does this on its development charts
but I have never seen a developer specified on a film box.
In any case, the calculator of an exposure meter is
designed to fit an average scene into the usable range of
the film. Changing the film speed simply moves the exposure
left or right along the curve. The mid gray value, which is
often argued, is actually of little relevance, provided its
in the linear part of the curve somewhere. More im****tant is
where shadows that are to have some detail fall. They must
be recorded on the film characteristic at a point where the
contrast is high enough to record the detail. If too far
down on the toe the shadows become blank. Moving them up
will improve their rendition but exposure must not be moved
up the curve enough to make the highlights _which are to
have detail_ fall onto the shoulder, where, again, the
contrast is low. For modern film there is no practical
shoulder unless the film is overexposed by a great many
stops.
Printing density is another consideration: overall
density becomes higher as exposure is increased. Negatives
which are so dense that they take very long printing
exposures are undesirable even if they deliver good tone
rendition.
Worrying about small errors in exposure is useless for
normal B&W, the im****tant thing is to give the negative
enough exposure for good shadow detail. Once this mimimum is
met there is a long range of increased exposure which will
still result in good tone rendition in the print.
--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]
|
