Exposure indexes and exposure

"Uranium Committee" wrote in
message
om...
From Kodak's book 'Kodak Films for Black-and-White
Photography', 1960.
The text was written before the change in ASA speeds.


EXPOSURE INDEXES

Kodak article snipped...
The big change came in 1958 when the ASA changed from
measuring using the Kodak speed method to the DIN method and
dropped the very large safety factor it had attached to the
Kodak method. The Kodak method and the DIN method come up
with very similar speeds but reducing the safety factor from
2.5 to 1.25 effectively doubled the speeds of all films on
the market which were measured by this method (not color or
reversal films or specialty films like X-Ray film). There
have been a number of changes in the ASA standard, now the
ISO method, over the years. Originally it specified a
developer to be used in the measurements. Then a standard
and a fine grain developer, neither of which was much like
actual developers used in practice. That was dropped in the
last revision of the standard which now allows the use of
any developer for measurement provided the developer is
stated in the results.
Kodak now uses the term Exposure Index or EI to mean
effective film speed when processed in some way other than
that used to get the ISO speed. Since negatives are often
developed to contrast indices other than the one produced by
the ISO method the effective speed will be different. For
instance, the ISO method results in a contrast index about
right for diffusion enlarging or contact printing. If
development is reduced to reduce the contrast to the right
amount for condenser enlarging the speed will be reduced
about 3/4 to 1 stop.
Kodak's explanation of practical exposure is a good one.
The extensive research they refer to is mostly that done by
Loyd A. Jones of Kodak Research Labs, over a thirty year
period. Jones is the one who came up with the Kodak Speed
method, which takes into account the minimum contast
necessary in the toe region of the film to obtain good
shadow detail. This proved too difficult to use routinely so
the fixed minimum density DIN method was adopted eventually.
Jones idea was to find the minimum exposure which would
result in good tonal rendition. The reason is that, in
general, grain and sharpness are optimized when the overall
density of the negative is minimal. Tonal rendition will
remain good for a very considerable range of exposure above
the minimum but grain will be increased and sharpness
reduced. Unfortunately, the large saftey factor adopted by
the ASA eliminated this advantage. Jones and his associates
at Kodak also photographed a large number of scenes of
different tonal characteristics. They measured the contrast
of the scenes and photographed them over a wide range of
exposures. They derived the minimum exposure criteria from
making the best prints possible from each negative and
presenting them to a very large group of judges who were
asked to pick the "first excellent print" from the series of
increasing exposures.
Jones published perhaps a dozen or more papers between
about 1920 and the early fifties, all worth reading.
Unfortunately, most of them are in somewhat obscure
scientific journals, like the Journal of the Franklin
Institute.


--
---
Richard Knoppow
Los Angeles, CA, USA

"Uranium Committee" wrote in
message
om...
From Kodak's book 'Kodak Films for Black-and-White
Photography', 1960.
The text was written before the change in ASA speeds.


EXPOSURE INDEXES

Kodak article snipped...
The big change came in 1958 when the ASA changed from
measuring using the Kodak speed method to the DIN method and
dropped the very large safety factor it had attached to the
Kodak method. The Kodak method and the DIN method come up
with very similar speeds but reducing the safety factor from
2.5 to 1.25 effectively doubled the speeds of all films on
the market which were measured by this method (not color or
reversal films or specialty films like X-Ray film). There
have been a number of changes in the ASA standard, now the
ISO method, over the years. Originally it specified a
developer to be used in the measurements. Then a standard
and a fine grain developer, neither of which was much like
actual developers used in practice. That was dropped in the
last revision of the standard which now allows the use of
any developer for measurement provided the developer is
stated in the results.
Kodak now uses the term Exposure Index or EI to mean
effective film speed when processed in some way other than
that used to get the ISO speed. Since negatives are often
developed to contrast indices other than the one produced by
the ISO method the effective speed will be different. For
instance, the ISO method results in a contrast index about
right for diffusion enlarging or contact printing. If
development is reduced to reduce the contrast to the right
amount for condenser enlarging the speed will be reduced
about 3/4 to 1 stop.
Kodak's explanation of practical exposure is a good one.
The extensive research they refer to is mostly that done by
Loyd A. Jones of Kodak Research Labs, over a thirty year
period. Jones is the one who came up with the Kodak Speed
method, which takes into account the minimum contast
necessary in the toe region of the film to obtain good
shadow detail. This proved too difficult to use routinely so
the fixed minimum density DIN method was adopted eventually.
Jones idea was to find the minimum exposure which would
result in good tonal rendition. The reason is that, in
general, grain and sharpness are optimized when the overall
density of the negative is minimal. Tonal rendition will
remain good for a very considerable range of exposure above
the minimum but grain will be increased and sharpness
reduced. Unfortunately, the large saftey factor adopted by
the ASA eliminated this advantage. Jones and his associates
at Kodak also photographed a large number of scenes of
different tonal characteristics. They measured the contrast
of the scenes and photographed them over a wide range of
exposures. They derived the minimum exposure criteria from
making the best prints possible from each negative and
presenting them to a very large group of judges who were
asked to pick the "first excellent print" from the series of
increasing exposures.
Jones published perhaps a dozen or more papers between
about 1920 and the early fifties, all worth reading.
Unfortunately, most of them are in somewhat obscure
scientific journals, like the Journal of the Franklin
Institute.


--
---
Richard Knoppow
Los Angeles, CA, USA

"Uranium Committee" wrote in
message
om...
From Kodak's book 'Kodak Films for Black-and-White
Photography', 1960.
The text was written before the change in ASA speeds.


EXPOSURE INDEXES

Kodak article snipped...
The big change came in 1958 when the ASA changed from
measuring using the Kodak speed method to the DIN method and
dropped the very large safety factor it had attached to the
Kodak method. The Kodak method and the DIN method come up
with very similar speeds but reducing the safety factor from
2.5 to 1.25 effectively doubled the speeds of all films on
the market which were measured by this method (not color or
reversal films or specialty films like X-Ray film). There
have been a number of changes in the ASA standard, now the
ISO method, over the years. Originally it specified a
developer to be used in the measurements. Then a standard
and a fine grain developer, neither of which was much like
actual developers used in practice. That was dropped in the
last revision of the standard which now allows the use of
any developer for measurement provided the developer is
stated in the results.
Kodak now uses the term Exposure Index or EI to mean
effective film speed when processed in some way other than
that used to get the ISO speed. Since negatives are often
developed to contrast indices other than the one produced by
the ISO method the effective speed will be different. For
instance, the ISO method results in a contrast index about
right for diffusion enlarging or contact printing. If
development is reduced to reduce the contrast to the right
amount for condenser enlarging the speed will be reduced
about 3/4 to 1 stop.
Kodak's explanation of practical exposure is a good one.
The extensive research they refer to is mostly that done by
Loyd A. Jones of Kodak Research Labs, over a thirty year
period. Jones is the one who came up with the Kodak Speed
method, which takes into account the minimum contast
necessary in the toe region of the film to obtain good
shadow detail. This proved too difficult to use routinely so
the fixed minimum density DIN method was adopted eventually.
Jones idea was to find the minimum exposure which would
result in good tonal rendition. The reason is that, in
general, grain and sharpness are optimized when the overall
density of the negative is minimal. Tonal rendition will
remain good for a very considerable range of exposure above
the minimum but grain will be increased and sharpness
reduced. Unfortunately, the large saftey factor adopted by
the ASA eliminated this advantage. Jones and his associates
at Kodak also photographed a large number of scenes of
different tonal characteristics. They measured the contrast
of the scenes and photographed them over a wide range of
exposures. They derived the minimum exposure criteria from
making the best prints possible from each negative and
presenting them to a very large group of judges who were
asked to pick the "first excellent print" from the series of
increasing exposures.
Jones published perhaps a dozen or more papers between
about 1920 and the early fifties, all worth reading.
Unfortunately, most of them are in somewhat obscure
scientific journals, like the Journal of the Franklin
Institute.


--
---
Richard Knoppow
Los Angeles, CA, USA

"Uranium Committee" wrote in
message
om...
From Kodak's book 'Kodak Films for Black-and-White
Photography', 1960.
The text was written before the change in ASA speeds.


EXPOSURE INDEXES

Kodak article snipped...
The big change came in 1958 when the ASA changed from
measuring using the Kodak speed method to the DIN method and
dropped the very large safety factor it had attached to the
Kodak method. The Kodak method and the DIN method come up
with very similar speeds but reducing the safety factor from
2.5 to 1.25 effectively doubled the speeds of all films on
the market which were measured by this method (not color or
reversal films or specialty films like X-Ray film). There
have been a number of changes in the ASA standard, now the
ISO method, over the years. Originally it specified a
developer to be used in the measurements. Then a standard
and a fine grain developer, neither of which was much like
actual developers used in practice. That was dropped in the
last revision of the standard which now allows the use of
any developer for measurement provided the developer is
stated in the results.
Kodak now uses the term Exposure Index or EI to mean
effective film speed when processed in some way other than
that used to get the ISO speed. Since negatives are often
developed to contrast indices other than the one produced by
the ISO method the effective speed will be different. For
instance, the ISO method results in a contrast index about
right for diffusion enlarging or contact printing. If
development is reduced to reduce the contrast to the right
amount for condenser enlarging the speed will be reduced
about 3/4 to 1 stop.
Kodak's explanation of practical exposure is a good one.
The extensive research they refer to is mostly that done by
Loyd A. Jones of Kodak Research Labs, over a thirty year
period. Jones is the one who came up with the Kodak Speed
method, which takes into account the minimum contast
necessary in the toe region of the film to obtain good
shadow detail. This proved too difficult to use routinely so
the fixed minimum density DIN method was adopted eventually.
Jones idea was to find the minimum exposure which would
result in good tonal rendition. The reason is that, in
general, grain and sharpness are optimized when the overall
density of the negative is minimal. Tonal rendition will
remain good for a very considerable range of exposure above
the minimum but grain will be increased and sharpness
reduced. Unfortunately, the large saftey factor adopted by
the ASA eliminated this advantage. Jones and his associates
at Kodak also photographed a large number of scenes of
different tonal characteristics. They measured the contrast
of the scenes and photographed them over a wide range of
exposures. They derived the minimum exposure criteria from
making the best prints possible from each negative and
presenting them to a very large group of judges who were
asked to pick the "first excellent print" from the series of
increasing exposures.
Jones published perhaps a dozen or more papers between
about 1920 and the early fifties, all worth reading.
Unfortunately, most of them are in somewhat obscure
scientific journals, like the Journal of the Franklin
Institute.


--
---
Richard Knoppow
Los Angeles, CA, USA

"Richard Knoppow" wrote in message ...
"Uranium Committee" wrote in
message
om...
From Kodak's book 'Kodak Films for Black-and-White
Photography', 1960.
The text was written before the change in ASA speeds.


EXPOSURE INDEXES

Kodak article snipped...
The big change came in 1958 when the ASA changed from
measuring using the Kodak speed method to the DIN method and
dropped the very large safety factor it had attached to the
Kodak method.

The booklet has a supplementary insert page which is dated May, 1960,
describing this change. The data sheets in the rest of the book refer
to the old ASA speeds, and is dated 'Seventh Edition, 1956. First 1958
printing'. So the change must have occurred between 1958 and 1960.
Kodak specifically mentions in the data sheets that the exposures you
get using the published ASA speeds are double the minimum needed.