On Oct 31, 5:38 pm, Peter Irwin <pir...@[EMAIL PROTECTED]
> wrote:
> Richard Knoppow <dickb...@[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.
> --
> pir...@[EMAIL PROTECTED]
is my opinion, and that of many others, that the 'new' ASA speed
sytem introduced in 1959 gives numbers that are too high, by about 2/3
stop.
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