[rollei_list] Re: ASA
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Fri, 19 Feb 2010 18:09:47 -0800
----- Original Message -----
From: "Don Williams" <dwilli10@xxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, February 19, 2010 2:46 PM
Subject: [rollei_list] Re: ASA
At 03:56 PM 2/19/2010, you wrote:
If I understood it right the ASA is based on a fixed
development
standard while the ISO allows a flexible development
standard,
which usually means the time needed. As I understood it,
the PMA
folks pushed for the looser ISO standard so color film
especially
could be marketed as 800, 1600 ISO. These film all require
longer
development times then slower films. My understanding is
that very
few films would be rated higher then 400 under the ASA
system. Any
thoughts? Ed A.
Sort of. The contrast is fixed. The standard calls for a
given range of densities to be produced by a given range of
exposure. This amounts to a fixed contrast index but is
independant of curve shape. While the developer can be any
desired the development time must be adjusted to produce the
required density vs exposure range. Actually, this was true
of the older versions of the standard that specified a
developer or developers and give formulas for them. The
development time is still whatever is required by the film
to yield the contrast specified by the method. This was also
true of the Kodak method and the original ASA standard based
on it. Even though the speeds changed by a factor of two
when the ASA adopted the modified DIN method the difference
was in the calculation not in the data from the test. The
ASA simply halved the speed given by the test. This was due
to the desire to insure that there would be an image on the
film and because Jones, et.al, had determined that print
quality was not degraded by an increase in exposure of
several stops. The resulting negatives tended to be a bit
dense. Kodak, in some if its film data sheets of the time
stated that an increase in effective speed of one stop was
permissible to reduce overall density provided the user was
sure of the exposure level.
Jones' original intension was to find the minimum
exposure for good tone rendition (first excellent print of a
series made from increasing exposure) because thin negatives
tend to have less grain and be sharper than denser ones.
This is less true of modern films than of the 1930s and
1940s films used by Jones but its still true to some degree.
Of course, this is of much greater consequence for 35mm
negatives than for larger ones.
The Kodak method requires measuring two gradients and
finding their ratio. The first is the gradient of the toe
region of the curve, the second is the straight line
portion. The speed point is where the toe has one third the
gradient (contrast) of the overall curve. This can be
difficult to measure. In fact, Kodak at one time published
an overlay to help find the two points.
The ASA decided to adopt the DIN method because it was
simpler requiring only that the densities be measured for a
series of controlled exposures. The speed point was where
the density was log 0.1 above the gross fog and base
density. The ASA modified this with a multiplication factor
that moved the speed point up from the log 0.1 point to a
higher point on the toe. They found in a survey of a great
many films of the time that the difference between this
point, a fixed amount higher than the log 0.1 point, was
very close to the point as determined by the Kodak/Jones
minimum gradient method and was a lot easier to measure
reliably. At the same time as this change was made the ASA
dropped the fudge factor so that all published film speeds
were essentially doubled. Again, this is not due to any
difference in the two methods but rather an artifical
difference due to a delibrate multiplication factor
introduced into the Kodak speed method. Speeds as
determinied by the Kodak method and the modified DIN method
are essentially identical.
Note that there was an earlier DIN method, based on
another principal, which was not very satisfactory. The
current DIN standard is identical to the current NIST/ISO
method in the US. Speeds can be reported as either
arithmetic or logerythmetric, calculations are included in
the standard for both.
To summarize, the contrast is fixed by the
specifications of the standard. It is about what would be
right for diffusion printing or contact printing. The
reported speed applies only when the film is developed to
this contrast. When some other contrast is required the
effective speed changes. A chart of speed vs: contrast is
given in the standard. By varying the amount of development
the contrast is changed with minimum change in the minimum
density on the film for reduced contrast or for the maximum
contrast where the contrast is increased.
In some cases, such as when using a condenser enlarger,
the effective contrast of the negative is increased due to
Callier effect. In order to print on the same contrast grade
paper as would be used for a diffusion light source, the
contrast of the negative must be adjusted downward. In
practice the reduction of development needed for this
results in a speed reduction of about 3/4 stop meaning that
exposure must be increased by that amount.
Of course, the same negatives can be printed on any
type of enlarger or printer provided the paper contrast is
adjusted accordingly. Dr. Richard Henry, in his book
_Controls in Black and White Photography_ shows overlayed
curves from condenser and diffusion enlargers where, on one
case, the negative contrast was adjusted and the same paper
was used in both enlargers, and in the other case where the
same negative was printed using paper of different grades.
The curves for both conditions lie exactly on top of each
other. Both techniques work and neither seems to be superior
although that may not hold where the difference in contrast
is several grades.
The developer makes no difference, with the exception of
special purpose developers such as strongly "compensating"
developers or very high contrast developers, both of which
produce strongly non-linear results. For "normal" developers
there really is no such thing as a low or high contrast
developer but active developers have an easire time of
producing high contrast and low activity ones for making low
contrast negatives. However, a developer like D-76 or D-23,
once considered low contrast and a developer like Dektol
will produce exactly the same contrast negatives if the
developing time is adjusted appropriately.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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