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"John" wrote in message
m...
Jean-David Beyer wrote:
John wrote:
Jean-David Beyer wrote:

I thought they did make a better B&W T-Grain film. It
TMax films are both
sharper, finer-grained, and have a straighter D/H curve
than any I have ever
used in sheet sizes, anyway.
That straighter curve has me puzzled. Doesn't it
compress luminance/density values? Maybe habit has
prejudiced me. I'll have to try some in 4x5 so that
perhaps I'll understand why some like it.

No, the toe of longer toe films compresses the shadows,
and the shoulder (if
present) in other films compresses the highlights. The
only film I ever used
that had a shoulder in the ighlights (in the useful
range) was Panatomic-X.

With a very short toe, you do have to be careful not to
underexpose because
if you do, you get nothing.

Please don't beat me up just yet. Do we actually use
either toe? If one were to take the actual ranges of
exposure that are generally useful to printing would they
include either toe? IOW, make a rectangle of the bounded
(useful) area and slide it up and down the curve. Toe? No
toe? OK, now you can beat me up.


The toe and shoulder are areas that have lower contrast
than the rest of the curve. The area in-between is called
the straight line portion even though its not really
straight in many films. The contrast of both shoulder and
toe change gradually, the toe becomming greater as it
approaches the straight line portion and the shoulder slowly
loosing contrast as it is approached. For many modern films
the range between the end of the toe and beginning of the
shoulder is more than ten stops but it also depends on the
developer, especially the shoulder end. Some developers do
not produce as high a maximum density as others.
There is a minimum exposure to get the shadows up far
enough on the toe to get good shadow detail. Many decades
ago a researcher from Kodak Research Labs named Loyd A.
Jones (that's the correct spelling) conducted extensive
experiments over a period of decades to determine the
optimum exposure of the negative for good tonal rendition on
the print. What he discovered was that the deepest shadow
that was to have any detail had to be at a point on the toe
where the gamma (contrast of a point) was about 1/3rd of the
contrast of the straight line section of the curve. Greater
exposure made little difference up to an increase of perhaps
ten stops but even a small reduction resulted in poor shadow
detail and a noticable reduction in print quality. The
judgement of print quality was done by double-blind testing
of prints made from actual scenes. I no longer remember the
number of observers but it was considerable and there were
hundreds of scenes of various sorts. From this data Jones
developed a speed measureing system. This was used
internally at Kodak for many years. Jones wanted to find the
_minimum_ exposure for good tone rendition because
relatively thin negatives have an advantage in looking
sharper and having somewhat less grain than denser ones.
This was more true of the thick emulsion films of the time
(1920s through 1940s) than it is now, but its still
considered good practice to make negatives on the thin side
provided the tone rendition is good. In 1943 the Jones/Kodak
system was adopted by the American Standards Association
(ASA) as its speed system. Unfortunately, the ASA decided to
add a one stop fudge factor to the speeds. This resulted in
excessively dense negatives. Probably the reason for the
fudge factor was an attempt to make sure that casual
photographers got a printable image even if it was not of
optimum quality.
In about 1958 the ASA adopted a different system. The
main reason was the difficulty of measuring speeds by the
Jones method. It required measuring two curves
simultaneously. The system which replaced it was a
modification of the second version of the German DIN system
(there was an older DIN system which was different). The DIN
system based its speeds on a certain minimum density above
gross fog and the base density. The ASA found, after
surveying nearly all the pictorial films on the market at
the time, that if a multiplier of 1.4 was added to the speed
needed to get a density of log 0.1 above fog and base
density the point would be, in virtually all cases,
identical with the one found by the Jones minimum gradient
system and would be a lot easier to measure consistently.
This new system was adopted and at the same time the fudge
factor was dropped. As a result the speeds of all films
jumped by a full stop! The current speed standard
promulgated by the ISO and, in the USA, by NIST, is a
ammended version of the 1958 standard. The main difference
is that no standard developer is specified. The measurement
can be made with any developer the film manufacturer wants
as long as that developer is stated with the results.
In fact, the new system still incorporates Jones' idea
of making exposures so that the darkest shadow to have
detail is still on a part of the toe having sufficient
contrast to print well. For many films many photographers
find that a small increase in exposure gives them better
shadow detail. This is probably due to variations in
exposure measurement, flare, and processing than in the film
or speed method.
Note that the speed measurement is made at what
ammounts to a fixed, standard, contrast. This contrast is
about right for contact printing and diffusion enlarging. If
some other contrast value is desired and a different
developing time is used to obtain it the _effective_ speed
of the film will change. The standard specifies a range of
exposure at the film surface which will translate to a
specified range of densities on the developed film.
Development during testing is adjusted to obtain this range
of densities. Of course, if development is varied the range
of densities will also vary, this is, by definition, a
change in contrast. Generally a lowering of contrast will
reqire more exposure and raising of the contrast will
require less. The ISO standard contains a chart for
calculating the variation in effective speed (sometimes
called exposure index) which results from this variation.
A note on the shape of film curves. Kodak's published
curves appear to be genuine and not approximations. They can
be compared and some idea of what the variations will do to
the reproduced gray scale can be seen. Some film and
developer combinations result in a slight hump in the mid
gray range and some films, notably ISO-350 Tri-X, have a
delibrately very long toe, meaning the gamma increases
gradually all throughout the curve until reaching extremely
high values of density (out of the usable range). If the
same scene is photographed on films of different curves and
the shadow and highlight points are chosen to be the same,
the effect will be that long toe films will tend to
reproduce the mid-grays darker and the humped films
reproduce them lighter, than in the original scene. A
stright line film will reproduce them linearly. All three
types have their places. The long toe film will tend toward
dramatic highlights where the humped film will tend toward
luminous gray values.
Of course, the same sort of variations occur in printing
paper but there the effect is the reverse of those from the
film. A paper with a long toe will tend to reproduce mid
grays brighter than one with a staight line.


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Richard Knoppow
Los Angeles, CA, USA

Thank you, Richard, for the cogent article. You have added to the body
of knowledge by placing 'the curve', in this case, into something I can
understand.

"John" wrote in message
m...
Thank you, Richard, for the cogent article. You have added
to the body of knowledge by placing 'the curve', in this
case, into something I can understand.

See if you can find an old Kodak film handbook
somewhere. These had a very good section on basic
sensitometry in them explaining how the curves are generated
and their relavance to the results. A similar article on
print sensitometry appeared in the _Professional Printing_
book. These sometimes show up in used book stores or maybe
eBay.
There are good summaries of Jones' work in the classic
_Theory of the Photographic Process_ C.E.K Mees, the
"revised edition is best for this. Jones' work was scattered
in several professional journals. I don't have a complete
bibliography but there are many citations in Mees's book.
More may be found in later editions.
When Mees founded the Kodak Research Laboratories in
1912 it was decided to publish technical and scientific
papers in established, peer reviewed, journals, rather than
in a house organ. This lent immediate credibility to the
papers but can make them hard to find. Much of Jones stuff
appears in the _Journal of the Franklin Institute_ and also
in the _Journal of the Optical Society of America_. For many
years Kodak published summaries of scientific and technical
literature in photography and related fields. There were
several and I can cite them if you like. I don't know of any
on-line source for these and the local libraries seem to
have burried them in dead storage. They are helpful if one
is doing serious research.


--
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Richard Knoppow
Los Angeles, CA, USA

"Richard Knoppow" wrote in message
m...

"John" wrote in message
m...
Thank you, Richard, for the cogent article. You have added to the body of
knowledge by placing 'the curve', in this case, into something I can
understand.

See if you can find an old Kodak film handbook somewhere. These had a
very good section on basic sensitometry in them explaining how the curves
are generated and their relavance to the results. A similar article on
print sensitometry appeared in the _Professional Printing_ book. These
sometimes show up in used book stores or maybe eBay.
There are good summaries of Jones' work in the classic _Theory of the
Photographic Process_ C.E.K Mees, [...]

Thank you. I have ordered the book from our state library. He wrote at least
two more books which we have in our university library. I have some reading
for the weekend!

"Geoffrey S. Mendelson" wrote in message
...
jch wrote:
I surely agree with that last comment. I really liked
Panatomic-X. It
made very good B&W slides as well. Would buy a batch of
100 rolls to
start with. Wonder what would be involved for Kodak to
swing a Tri-X
line around to make a run of Panatomic-X? Is it just the
emulsion, or
the emulsion /and/ the base?

I expect that the base made today would be very different
than the base
used in Pan-X, but IMHO it would not matter. As long as it
was a reasonably
clear base (remember the Efke dark grey base?), it's to me
the emulsion
that would count.

If it's really Lucky and not Kodak that made it for $2.00
a roll, lots of
people would buy it. I fit were Kodak themselves and they
had to charge $10
a roll, a lot less of them would be sold.

Geoff.

The same sort of base is used now and would make no
difference to the emulsion. For reversal purposes it would
be good to use a clear base with the under the emulsions
type of anti-halation coating used for color film. The same
equipment is used to coat different films at various times
although I am sure the color line is not used for B&W. Film
and paper is made in batches which are stored until needed.
I have no idea how much of anything Kodak would find
economical to make. I suspect these days it would be quite a
bit. I wonder if they even remember how to make Panatomic-X.
Note, 35mm negative B&W film is often coated on a
support which has a pigment incorporated into it. The
pigment is not removable in processing. The purpose of the
pigment is to prevent "light-piping" or the tansverse
transmission of light in the support. The reason for this in
35mm film is because one end of the film is often exposed to
light when loading. Light can be conducted some way into the
film causing fogging. This pigment is in addition to the
anti-halation dye in the back coating. Anti-halation dye is
to prevent reflection of light from the back surface of the
support. Since the anti-halation coating is on the back of
the film it can not act to prevent light-piping so both are
needed. Many color films have an anti-halation coating on
top of the support and under the emulsion. This is very
effective in stopping both light-piping and halation since
light conducted through the support can not reach the
emulsion and no light can get through the coating to be
reflected. I don't know why this system is not used for B&W
film. It may be that the dye in the coating is removed or
decolorized by one of the bleach steps where the dye in B&W
is converted to a colorless form by the sulfite in both
developer and fixing bath.


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Richard Knoppow
Los Angeles, CA, USA