[pure-silver] Re: Acros & HC-110
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 17 Feb 2006 15:12:41 -0800
----- Original Message -----
From: "Mike" <bw.zone@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, February 17, 2006 1:16 PM
Subject: [pure-silver] Acros & HC-110
Hello all,
I'm going to be doing some film / dev. testing this
weekend. Fuji
100 Acros 120 with HC-110 to be exact. What I wanted to know
is,
should I still test for my speed point at .15 + base plus
fog, or has
that changed with the newer films? I thought I had read the
the t-max
films had a different speed point.
Also if anyone has any comments about this film / dev.
combo please
share your experiences pro and con. Or any other developer
one might
have used.
Thanks in advance for any help.
Mike
The ISO speed for B&W still films (there are other
standards for other kinds of film and for other
applications) is based on a fixed contrast. The standard
states a range of exposure and the range of densities to be
produced from it. The standard prescribes an exposure range
of log 1.30 for a density range of log 0.80. This is a G-bar
of about 0.62, which is suitable, with most films, for
contact printing and diffusion enlarging. Density is to be
measured with a Type 2 diffusion densitometer.
The low density point for the measurement of both
exposure and density is where the density is log 0.01 above
base density plus gross fog. This point is designated Hm.
The calculation for speed is based on this point:
For arithmetric speeds:
S = 0.08/Hm where Hm is in Lux seconds
Where H is in Lux-seconds
Note that changing the contrast of the image will also
change the effective film speed. Speeds derived from
conditions other than those specified in the standard are
called Exposure Index by Kodak.
The 0.8 multiplier is intended to move the speed point up
the toe of the curve so that the minimum contrast for shadow
exposures is sufficient to show detail.
Some history:
The original ASA speed system, adopted in 1943 and used
until 1958, was based on work done by Loyd A. Jones and his
associates of Kodak Labs. The system was adopted internally
by Kodak in about 1940 and speeds published as "Kodak
Speeds".
Jones speed system was based on extensive research of
tone reproduction in practice. In addition, there was a
criteria of having the minimum exposure which resulted in
excellent tone reproduction. This was because grain and loss
of sharpness are both minimized by keeping the density low.
Jones called his system the minimum usable gradient system.
The minimum being where the gradient of the toe was 1/3rd
that of the overall average curve.
When the ASA adopted the Kodak method they added a
substantial safety factor, of 2.5X. This resulted in overly
dense negatives. In fact, Kodak, in their instructions noted
that film speeds could be doubled if care was used in
exposing and processing. The Jones system turned out to be
very difficult to measure in practice so, in 1958, a
different system was adopted. This was based on the existing
German DIN system (the second generation, the original DIN
system was something else). The DIN method was based on a
fixed density point. The ASA did extensive research
surveying the negative materials available at the time and
determined that the minimum gradient point of nearly all was
at a fixed distance from the minimum density point within a
small variation. The 0.8 factor in the speed calculation is
to move the speed point to the minimum density point.
The new method was adopted by the ASA in 1958, the former
safety factor was dropped in the new standard, immediately
doubling all films speeds.
Originally, the standard specified the developers to be
used in testing. But, because these were not standard
developers the speeds were often not quite what was obtained
in practice. The current ISO standard does not specify a
developer whatever developer is used by the manufacturer
must be stated along with the speed. Variations in
developers can result in speed variations of about + or -
3/4 of a stop, not too great.
Variations of contrast with the same developer can result
in about a + or - 3/4 stop change for a change in contrast
approximating + or - one paper grade.
In some cases the desirable contrast of different films
for printing on the same materials may be different. For
instance, Kodak does recommend slightly lower Contrast
Indexes for some T-Max films than for conventional
emulsions. The data sheets show this although it may take
some searching around to find it. Presumably, Kodak's
exposure and development recommendations reflect these
differences.
Ilford reported some time ago that it does not use the
ISO standard for speed measurement. I think (but don't know
for certain) that the variation is that they use a lower CI.
Ilford uses a compromise CI between that suitable for
diffusion printing and condenser printing. The difference
between effective contrast for the two types of illumination
for the same negative is about one paper grade (assuming the
usual partially diffusing condenser system, such as those
found on Omega and Bessler enlargers). So, Ilford splits the
difference allowing "normal" prints to be made on grade-2
paper on either system.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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