"Peter Irwin" <pirwin@[EMAIL PROTECTED]
> wrote in message
news:fgaslh$lvk$1@[EMAIL PROTECTED]
> Richard Knoppow <dickburk@[EMAIL PROTECTED]
> wrote:
>> For peter, the 0.8 factor is NOT the same as 1, it is
>> 20%
>> different which is quite different. The density value I
>> stated is the value from the charts in the standard for
>> the
>> log exposure at the speed point multiplied by the
>> reciprocal
>> of 0.8, that is 1.25 and should be the log exposure at
>> the
>> point on the toe approximating the Jones minimum gradient
>> point. The 0.8 factor is in the standard and it used to
>> calculate the speed.
>>
>
> I think this is getting a little confusing for me.
> I will state what I understand as clearly as possible,
> and you can tell me if I'm going wrong somewhere.
>
>
> Kodak Speed
> -----------
> In 1939 Kodak introduced a new speed system based
> on the results of extensive psychophysical research.
> This research showed that the minimum useful exposure
> Required to yield an 'excellent' print was at a point
> where the gradient of the H&D curve was 0.3x times the
> average gradient of the slope over a range from the
> exposure point to log 1.5 above the exposure point.
>
> Finding this point called the "Jones Point" requires
> a recursive operation. One has to first guess the point
> to find the average gradient, and then refine your guess
> on the second try.
>
> Kodak speed is given by the formula:
>
> Kodak Speed = 1/E
>
> Where E is the Jones Point in Metre-Candle-Seconds.
> (Same as Lux Seconds).
>
> If you had such a thing as a exposure meter
> calibrated for Kodak speeds, there would
> be no safety factor. Kodak Speed didn't catch
> on because few if any people owned such meters.
>
>
> OLD ASA Speed 1943
> ------------------
>
> In 1943 the ASA adopted the Kodak speed system
> with one im****tant change.
>
> The formula was now
>
> ASA Speed = 1/(4 x E)
>
> This was intended to give numbers usable with
> both Weston and GE meters. The new ASA standard
> meters were to be calibrated midway between the
> old Weston and GE calibrations. With an ASA
> meter, old ASA speed had a safety factor of
> 2.5 (1 1/3 stops).
>
> I don't think there was a particular level
> of negative contrast required by the old ASA
> standard, because the Jones point remains in the
> same place over a fairly wide range of development
> contrasts. Development contrast is supposed
> to be typical of photofini****ng practice.
>
> DIN Speed
> ---------
> The original DIN speed system (1936) was
> very unsatisfactory. Sometimes it would
> even get the relative speeds of films
> in the wrong order. The reason for this
> was that the original DIN standard required
> films to be developed for maximum speed
> rather than according to normal use.
> Old DIN speeds are indicated by the presence
> of "/10" so 24/10 degrees DIN is an old
> DIN speed.
>
> In the 1957 DIN standard, the "optimal
> development" was replaced by a rigidly
> specified development more typical of
> real practice. People soon noticed that
> the new DIN numbers actually made sense.
>
> New ASA Speed
> -------------
> Since the new DIN system was easier for
> film manufacturers in practice and
> actually worked pretty well, the ASA
> decided to adopt a new system based on
> DIN speeds. It was also decided to
> abandon the rather large safety factor
> and have speeds roughly twice that of
> the old while keeping meter calibration
> unchanged.
>
> The 2:1 relation****p is typical, but is
> not exactly true of all films as the following
> table shows.
>
> Film OLD ASA NEW ASA
> ---- ------- -------
> Plus-X 35mm 80 125
> Verichrome Pan 80 125
> Tri-X Pan Sheet 200 320
> Tri-X Pan 35mm 200 400
>
> ISO Speed
> ---------
> The current ISO standards for B&W film
> are essentially the same as the New DIN
> and New ASA standards.
>
> Where Hm = the 0.1 above base + fog point in lux seconds.
>
> Arithmetic speed S = 0.8/Hm
>
> Log speed S degrees = 1 + 10 x log10 0.8/Hm
>
> The Log ISO is in fact the same as the
> DIN speed.
>
> DIN Speed = 10 log10 1/Hm
>
> Because 10 log10 0.8 = -1
> (at least very very nearly).
>
> So Log ISO and DIN are both equal to
> the log exposure of the 0.1 above base + fog point
> divided by minus 10.
>
> The 0.8 denominator in the arithmetic speed
> makes 1 ASA equal to 1 DIN and places the
> New ASA or ISO arithmetic speed where it
> was wanted.
>
> example:
> Tri-X pan 400
> ISO 400/27 degrees
> New ASA 400
> New DIN 27
> OLD ASA probably still 200
>
> 0.1 above base fog point (Hm) is -2.7 log lux seconds
>
> This is true from all formulas.
>
> ISO Artith or New ASA = 0.8/Hm
> = 0.8/10^-2.7
> = 0.8/0.002
> = 400
>
> ISO Log Degrees = 1 + 10 x log10 0.8/Hm
> = 1 + 10 x log10 0.8/10^-2.7
> = 1 + 10 x log10 400
> = 1 + 10 x 2.6
> = 1 + 26
> = 27
>
> DIN Speed = 10 x log10 1/Hm
> = 10 x log10 1/10^-2.7
> = 10 x log10 501
> = 10 x 2.7
> = 27
> Same Answer!
>
> DIN Speed of 27 means Hm is -2.7 log lux seconds.
> - neat - all you have to do is divide by negative 10.
>
> If Old ASA 200 then Jones point E = - 2.9 log lux seconds
>
> ASA Speed = 1/(4 x E)
> = 1/(4 x 10^-2.9)
> = 1/(4 x 0.00126)
> = 1/0.00504
> = 198.41
>
> Close enough.
>
> Peter.
> --
> pirwin@[EMAIL PROTECTED]
>
This is getting long but I can't find anything to snip.
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.
The ASA adopted the system in 1943 with a safety factor
of 2. ASA speeds were 1/4 of Kodak speeds. The resulting
number could be used with either Weston or General Elecric
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.
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). However,
they wanted to accomplishe two things: first was to make the
speeds compatible with earlier ASA speeds, and secondly, to
maintain something like the Jones idea of the minimum
gradient. 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. 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. So, a factor of
0.8, the reciprocal of 1.25 is introduced into the
calculation of the speed in the new ASA method. In effect
the speeds were now double those measured by the old ASA
method and half of the Kodak speed.
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.
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.
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 catagory is about the
same and its intended use is about the same so speed
comparisions must be made with some care.
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.
--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]
|
