Re: [Rollei] Rolleiflex Exposure Chart Error?

[Richard Knoppow <dickburk >]

At 04:13 PM 11/05/2001 -0800, you wrote:
>
>--- Rich Lahrson <tripspud  > wrote:
>>      Now, is this only true on the T? Do other
>> models show the
>> correct progression?
>
>Rich,
>     I suspect what you are seeing is the progression
>chart of the "old" ASA. There were two such, one
>supplanting the other, with less of a "fudge factor"
>built into the film speeds of the latter. It had been
>assumed that the user was less than ideally
>experienced in the use of meters and in how to apply
>the film speed properly, therefore, most films were
>under-rated in speed to insure that the films didn't
>end up under-exposed (a very common occurence, back
>then). The change-over, as I recall, was sometime in
>the early '50s or so, and I am sure Rollei wasn't
>about to recall all their unsold cameras to change the
>plates, sooo, most, if not all made with the old ASA
>charts got to market even long after the change. Has
>anyone explored this correlation before? If so, does
>it coincide with the accepted serial number
>progression, specifically, the time of the change and
>the accepted date of camera/lens manufacture? It would
>be interesting to see if they agree. Of course, it
>could just be a collosal waste of time and energy,
>too.
>Just some idle noodling.
>
>Jon
>from Deepinaharta, Georgia 
>
  The first ASA speed system was an adaptation of the Kodak system worked
out by Lloyd A. Jones of Kodak labs. Speeds in Kodak numbers were published
by Kodak beginning about 1940. 
  Jones system was based on long research on how much exposure was needed
for an "excellent print". Some very large double blind tests were done to
judge what consitituted an excellent print. 
  Jones system was based on a a minimum gradient in the toe. This point was
measured on the basis of the toe contrast rather than on a fixed minimum
density. The minimum was the point where the gradient or gamma of the tow
was 1/3rd that of the average gradient over a log 1.5 density range. So,
the system took into account the shape and extent of the toe region. 
  This tended to set exposure at a level where adequate shadow detail was
produced at a minimum exposure. 
  The reason for setting minimum exposure was that grain is reduced and
sharpness is increased over negatives with greater exposure. Jones found
that increasing exposure from this point had little or no effect on tonal
rendition up to very large overexposure.
  When the Kodak system was adopted by the ASA a 2.5X safety factor was
included. The reason given in Kodak literature is that it was to compensate
for underdevelopment in photofinishing plants, evidently very a very common
problem at the time. Probably the idea was that, since overexposure by this
much had no effect on print quality, it would insure that an image was
gotten by amateur photographers. This system was adopted by the ASA in
about 1943 and modified somewhat in 1946. The safety factor tended to
result in excessively dense negatives. Kodak's literature of the time
specifies that speed can be doubled without loss of quality when exposure
and processing is done carefully. 
   Kodak speeds do not have the safety factor. Old Kodak speeds translate
into modern speeds by dividing by two, and into the old ASA speeds by
deviding by four. 
  The Jones minimum usable gradient method of speed measurement is very
difficult to do reliably. In about 1958 the ASA adopted the DIN method of
speed measurement. This is essentially the system in current use. 
  The DIN system is based on a fixed minimum density (Log 0.1 above fog and
base density) and is based on a fixed gamma. The 1958 system had an 1.5X
safety factor effectively doubling the speeds of all films. 
  The ASA did a survey of films made at that time to determine the
difference in measureed speed between the DIN system and the Kodak system.
They decided that the differences were too small to warrent the substantial
additional difficulty of the Kodak system measurements. 
  Since that time there have been a number of changes in the ASA, later ISO
system. The original specified a developer, the formula for which was
included in the standard. The current version does not specify a developer.
The manuacturer can use any developer desired but it must be specified with
the resulting speeds. 
  Since the ISO system is based on a fixed average gradient it is not valid
for other degrees of contrast. The speed will be found to go down when the
film is developed to a lower contrast and to go up when developed to higher
contrast.
  This system is used only for B&W negative films for still cameras.
Reversal film, color film, motion picture films, all have separate standards.
  Its interesting that c.1946 there were a lot of "magic" developers
advertised which claimed to increase film speed by two stops.Well, they
did, but so would any good developer. Based on the original ASA speed they
could double or quadriple speed because the film was already a stop faster
than the speed indicated plus all films have about a one stop underexposure
latitude from the speed without a safety factor. All of these developers
disappeared quickly after the second ASA system, without the large safety
factor, was adopted. 
  Now, progression the original ASA speed, Kodak speeds, Weston speeds,
General Electric speeds, are all arithmetic so the numbers are the same.
ASA or current ISO speeds can be used on old meters by knowing the off-set
to use on the calculator. For Weston meters use the next lower number, for
old GE meters use the next highest number. GE adopted the ASA method in
1946 so even their older meters work on it. Weston continued to use their
own method for some time, don't have a date. I think the Weston Master III
is the first one with ASA speeds. 
  Older meters with ASA speeds use current ISO speeds, the correction is in
the speed number not the calulator. However, its easy enough to check using
the "sunny 16" rule.
  The DIN sytem can be expressed in either arithmetic or log values. Log
values are common in Europe, arithmetic values in the USA and UK. 
  There have been a very large number of speed measuring methods proposed
and actually used over something over a century. The first reliable meters
used small bits of printing out paper. The exposure was made for a fixed
time and the paper compared with a standard chart. The next step was the
photoelectric meter, introduced about 1931. There is some controversey over
who exactly marketed the very first but the first one to become popular was
the Weston c.1935. Probably because Weston also introduced a workable speed
measuring method along with the meter and supplied the film speeds
themselves rather than relying on the manufacturers. These meters were easy
to use and had logical calculators. However they were rather expensive so
it took some time, and cheaper models, for them to become wide spread in
use. 
- ----
Richard Knoppow
Los Angeles, CA, USA
dickburk  

------------------------------