I've written long pieces on this to this list and to the old
usenet list.
The "old" ASA speed system was one developed at Kodak Labs
mostly by Loyd A. Jones. It was based on the idea that negatives
should be of minimum density consistent with good tone rendition,
AKA, shadow detail. Jones decided on this after a very extensive
program of research based on obtaining "excellent" prints from
the negatives. The method turned out to be difficult to measure
in practice. It required measuring points at two degrees of
gamma, one, in the shadow (toe) area of 1/3rd the other. The
system was initially adopted in house by Kodak and called "Kodak
Speeds". These were published in the 1940s for many Kodak films
and are about four times the current ISO speeds. Among other
things, the speeds were such that underexposing about one stop
was the maximum limit if good tone rendition was to be gotten but
overexposure could be extended many stops without damaging the
quality of the print. Minimum exposure, or thinner densities,
were thought desirable because, in general, grain was minimized
and sharpness maximized. The lower densities tended to reduce
light scatter in the emulsion.
In the mid 1940's (I've forgotten the actual date) this
system was adopted, with some changes, by the ASA. The main
change was in adding a fudge factor of one stop. This was due to
the finding that increasing exposure, even by several stops, did
not damage the quality of the print but it was also desired to
insure a printable image was obtained. So, when the ASA adopted
the system the one stop fudge factor was included. In its film
data sheets, at least for professional films, Kodak advised that
when one had good control of lighting and processing film could
be shot at about one stop faster than the data sheet gave. The
thinner negatives would be easier to print.
A little over a decade later, the ASA decided that the method
was too difficult to measure, especially for films like Kodak
Plus-X Pan Professional sheet film, which had virtually no
straight line section in the characteristic curve. This film and
some others, were called "all toe". At this time the German
standards organization who published DIN standards, devised a new
film speed method. There was a much older method also called the
DIN standard but this one measured the speed in a different way.
The DIN method required measuring only at a single gamma value so
was much easier than the Jones method. It was also discovered, by
means of a great meany measurements on commercial films that the
speed point of the of the DIN method was the same as that found
by ASA/Jones if an offset was applied. This was not the same as
the old fudge factor and served a different purpose.
The ASA adopted the new method with the offset speed point
but did not continue the fudge factor. Since the speeds found by
the "new" ASA/DIN method were the same as the original Jones
method, the only difference in the published speeds was the
elimination of the fudge factor. So, all film speeds doubled
overnight! Now, because the speed of the original ASA method was
artificially lowered there were many "magic" developers sold on
the basis that they would double or even quadruple film speed. Of
course, they in fact had no effect what ever. The old speed could
be doubled by eliminating the fudge factor and that speed could
be pushed a stop because of the inherent margin of the film.
It should be pointed out that both the old and new ASA
speeds apply only to B&W negative film. There is a different
standard for reversal films, motion picture films, color films
both negative and reversal, because there is less margin in these
processes. For instance, motion picture negative film is meant to
be printed onto relatively high contrast positive material for
projection, where the dynamic range of the film (there is a
better term for this that I am drawing a blank on) goes from very
dense shadows to virtually clear highlights. To some degree this
is also true of reflection prints but the overall contrast is not
as great. As a result negative film for motion pictures has a
somewhat different speed than the same film for still photography.
I have the ISO standard for B&W negative still film but not
for the others so can't describe the difference in detail.
Unfortunately, while the standards are available from NIST they
are quite expensive.
One relatively late change in the ISO/NIST standard was the
elimination of a standard developer. Originally two developers, a
"normal" and a "fine-grain" type were specified. Eventually, both
were eliminated the test being valid for any developer provided
it is specified along with the resultant speeds. The ISO standard
also allows processing to a variety of gamma values with means of
calculating the variation in speed. In general, a one paper grade
difference in contrast produces about a 3/4 stop variation in
speed, increased speed for increased contrast and decreased speed
for decreased contrast, the resulting density range remaining
about constant.
BTW, I looked at the Kodak speeds in a 1940 "Photo-Lab-Index"
and found many of the films with familiar trade names had very
similar speeds to their current versions. I suspect the speeds
are set as a matter of convenience in using while the same speed
films now probably a significantly finer grain and sharper.
Enough already, my carpal tunnels are protesting.
On 9/29/2021 6:35 PM, Tim Daneliuk (tundra) wrote:
I posted this the UK 5x4 forum this evening, but thought others here might
find this noteworthy:
ASA or DIN are merely difference scales and can be directly converted one,
from the other, much like kilograms to pounds. Up until the early 1960s,
film ASA were about half of what they are today. The philosophy was "what
is the exposure that will yield a good image?" Sometime thereafter, film
speeds magically doubled because the measurement standard was changed to
"What is the minimum exposure that will render an image on the film?" This
persisted for both ASA and DIN. It became more like a mileage estimate on a
new auto - not absolutely accurate in actual practice, but useful for
comparing one film to another - an ASA 400 film can reasonably be expected
to be 1 full stop more light sensitive than an ASA 200 film.
This - in large part - is why the "real" ASA of films turns out to be 1/2
box speed - the way ASA (and DIN) are calculated is not realistic for real
picture taking.
What all this extended low agitation gibberish does (among other things) is
so fully develop the shadows that you DO get good images at the full rated
box ASA.
One way to think about this is - if you do extended development -
it will add a full stop of light sensitivity to the film. The exposure to
'get a good image' is now the same as the box ASA that was originally
computed for 'enough exposure to just show an image." I suspect that had
Kodak, Agfa, Ilford, et al actually designed their films for extended
development, they'd have marked the box ASA as 2x what we see today.
Incidentally, this is NOT the same thing as push processing as usually
construed. Push processing involves significantly underexposing and then
somewhat overdeveloping which usually gives you grainy, low contrast images
with harsh highlights. The film isn't actually being developed long enough
to realize the target ASA and the full strength developer is cooking the
grain. In any case, there is a limit to how much you can increase
ASA with development. David Kachel has written eloquently on this topic.
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