"Peter Irwin" <pirwin@[EMAIL PROTECTED]
> wrote in message
news:fg5ogt$tnv$1@[EMAIL PROTECTED]
> Richard Knoppow <dickburk@[EMAIL PROTECTED]
> wrote:
>
>> Assuming
>> the minimum usable gradient is somwehere around log -2.5
>
> I had been under the impression that the Jones
> point for a 200 old ASA film was at log -2.9.
>
> I'm not really reliable at math, so I'd be glad
> if someone would show me if I've gone wrong.
>
> If the old ASA speed definition is:
>
> old ASA speed = 1/(4xE)
>
> then E = 1/(4x old ASA)
>
> E = 1/(4 x 200)
> E = 1/800
> Log (E) = -2.903
>
I think you are refering to log exposure. I am going to
have to research this because I don't remember if the
ASA/Jones method used the same units (lux seconds) as are
currently used. In any case, the Jones method was based on
an exposure giving a minimum toe gradient of 1.3rd of the
overall straight line gradient. I think the ASA standard
required a contrast index about the same as the current ISO
standard, that is, approximately right for contact printing
or diffusion enlarging. I just looked at the Kodak curves on
their web site and estimated there the log exposure
intercepted the toe. In any case, moving a little one way or
another would not make much difference in overall exposure
range. You are probably right that the exposure I found for
these ISO-400 films are about what you will find for ASA-200
films due to the 2X safty factor the ASA applied to Kodak
speeds in its 1943 standard.
The current ISO method includes a multiplier of
1.4x. This is not a safety factor but rather to adjust the
speed gotten from the exposure for a density of log 0.1
above gross fog and sup****t density to the point where the
toe gradient is about 1/3rd the overall straight line
gradient. When the ASA adopted the new DIN method to replace
the Jones/Kodak method in 1958 it found by extensive survey
that for nearly all pictorial films the ratio between the
log 0.1 density point and the Jones speed point was about
this ratio, so the DIN speed is divided by 1.4 to obtain the
equivalent Jones/Kodak speed. The reason for changing the
method was that the Jones method proved very hard to use in
practice.
Jones and his associates at Kodak Labs conducted
extensive experimentation to find the practical mimimum
exposure needed by a film to deliver good tone rendition. He
found that the minimum shadow exposure had to be on the toe
no lower than where the toe gradient (or contrast) was 1/3
of the straight line gradient. Increased exposure had little
effect on tone rendition but perceived quality fell off very
quickly if the exposure was reduced from this point.
Jones chose to find the _minimum_ exposure because
films of the time had optimum grain and sharpness when at
minimum density. Modern films still have this property but
its much less now than in the period of the 1920's through
1940's when Jones did his work.
Kodak's approach to tone rendition was to standardize
the development of the negative so that a constant contrast
was obtained and to adjust print contrast, if necessary, by
choice of paper grade. This is about the opposite of the
Zone System which attempts to adjust negatives so that they
all print on the same grade of paper.
--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]
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