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]
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