Contrast Index Question: Newbie

Can someone explain to me contrast indexes. Are they published for each B&W
film, or do I have to calculate it by myself? TIA.

"In The Trenches" wrote in message news:BqMsc.588797$Ig.273737@pd7tw2no...
Can someone explain to me contrast indexes. Are they published for each B&W
film, or do I have to calculate it by myself? TIA.

'Contrast Index' (CI) is a measurement of contrast of B&W film, and
varies with the degree of development. Kodak's times given in their
instructions usually produce a CI of about 0.57, which is about
optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser
enlargers, and is obtained by reducing the developing times given by
about 25%.

You don't calculate this number or use it at all. It is simply for
measurement purposes.

Thanks, a few more questions though, how is the CI derived?. What do I use
to measure it? What does the number actually mean? Is there a chart
somewhere? I've got a condenser head on my MX45 and I've just been
developing as per their instructions on the box of film, so probably, I've
been doing everything at .57. Could I correct this by reprinting using my
vari contrast filters or will that make the problem worse? Thx.


"Michael Scarpitti" wrote in message
om...
"In The Trenches" wrote in message
news:BqMsc.588797$Ig.273737@pd7tw2no...
Can someone explain to me contrast indexes. Are they published for each
B&W
film, or do I have to calculate it by myself? TIA.

'Contrast Index' (CI) is a measurement of contrast of B&W film, and
varies with the degree of development. Kodak's times given in their
instructions usually produce a CI of about 0.57, which is about
optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser
enlargers, and is obtained by reducing the developing times given by
about 25%.

You don't calculate this number or use it at all. It is simply for
measurement purposes.

One more thing. If I'm reducing dev times should I also be rating my film
down too. i.e. 400-320 and 125 down to 100? Or what happens in this
instance.

"Michael Scarpitti" wrote in message
om...
"In The Trenches" wrote in message
news:BqMsc.588797$Ig.273737@pd7tw2no...
Can someone explain to me contrast indexes. Are they published for each
B&W
film, or do I have to calculate it by myself? TIA.

'Contrast Index' (CI) is a measurement of contrast of B&W film, and
varies with the degree of development. Kodak's times given in their
instructions usually produce a CI of about 0.57, which is about
optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser
enlargers, and is obtained by reducing the developing times given by
about 25%.

You don't calculate this number or use it at all. It is simply for
measurement purposes.

"In The Trenches" wrote in message
news:BqMsc.588797$Ig.273737@pd7tw2no...
Can someone explain to me contrast indexes. Are they
published for each B&W
film, or do I have to calculate it by myself? TIA.

Contrast Index, or CI, is a method of specifying contrast
devised by Kodak to be applicable to a large variety of
materials.
Contrast is the slope of the characteristic curve. This
curve shows the relationship between exposure and density.
It varies with development: the more the film is developed
the steeper the curve will be and the higher the contrast.
At one time contrast was defined as the slop of this curve
at a point on the straight line portion. Actual film curves
vary in slope from low to high densities. The difference in
slope is determined by the way the emulsion is made and
represents the distribution of sensitivity of the silver
halide particals which make up the emulsion. The shape of
the curve is decided on by the manufacturer to yield a
certain tonal rendition. The curve has three parts: the toe,
the straight line, and the shoulder. The toe is the low
density part. In general this has a lower contrast than the
straight line portion. The shadows are recoreded in this
region. The straight line portion represents most of the
film's recording range. It records differences in brightness
as a linear change in density. This is the region used for
most of the brightness information in the photo. The
shoulder is the high exposure- high density part of the
curve. Again, the contrast is lower than in the straight
line portion. Most modern films have such a long range of
densities that they never reach the shoulder in normal use
but many older films did resulting in "blocked" highlights.
Until about the 1940's contrast was usually measured as
"gamma" Gamma is the contrast of the straight line portion
of the curve. However, most films record part of the image
in the toe region and many films have relatively long toe
sections. Some, like Tri-X sheet film and the discontinued
Plus-X sheet film, have no straight line portion, the
contrast increasing with density at all values. For films
like this measuring contrast as gamma can be very misleading
about the printing quality of the negative. For instance, a
long toe film, like Tri-X Pan sheet film, when compared to a
short toe film, like T-Max 400, will have much lower
highlight density when developed to the same gamma. One
solution to this was the development of Average Contrast, or
bar-G (i.e., the letter with a dash over it to indicate its
an averaged value). Average contrast is, essentially, a
straight line drawn from the minimum density to some maximum
density. For the ISO method of measuring film speed bar-G is
defined indirectly by the specified exposure range and
expected resulting density range. For this method the range
of exposure is log 1.30. The film is developed so that the
density resulting from the maximum exposure is log 0.9 above
base density plus fog. The speed is measured between this
point and a minimum of logd 0.1 above base density and fog
for a density range of log 0.8.
Contrast Index is similar to bar-G but it specifies a
minimum density of log 0.2 above base and fog and is the
slope measured from this point to a high density point
corresponding to a range of LogE 2.0 from the exposure
needed to produce the low density reference. This is a more
realistic range of exposure in normal photography and the
reference point of logD 0.2 above base and fog is far enough
up the toe to eliminate problems with very low contrast toe
regions (i.e., no shadow detail). Contrast Index is
applicable to all sorts of film, including special purpose
films, like high contrast copy or graphic arts films.
While CI can be determined from a film curve it is easier
to measure using a graphic overlay designed by Kodak Labs.
These used to be available from Kodak but have probably been
discontinued or, perhaps, are available from Silver Pixel
Press.
CI was originally described in a paper published by the
Kodak Research Labs.

"Contrast Index" C.N.Nelson and J.A.C.Yule, _Photographic
Science and Engineering_ V. 10, No. 1, January-Febuary 1966

It is also described in detail in several books on
photographic science.

I should clarify that two negatives on different films
developed to the same contrast index will not look the same.
The curve of the film has a strong influence on tonal
rendition, however, at the same CI both negatives will print
on the same grade of paper and yield reasonably good prints.
Where they are developed to the same _gamma_ they will not
print on the same paper grade and the long toe film may have
inadequate shadow detail on any paper.

Kodak uses CI for its contrast specifications. Agfa seems to
still use gamma, and Ilford and others use bar-G.


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

Of course you can calculate it. But it is not simple. You calculate it by
contact printing a step wedge on your film and developing to whatever time the
manufacturer recommends or to the time you want. Now you measure the density
of the wedge images and plot the film curve and then use the definition of
contrast index to calculate what you got.

It is not easy but it is doable.

Kodak gives CI values for its recommended developing times but they are for
specific types of enlargers (like the OP said, diffusion enlargers) and you
may end up with a negative that is too contrasty if you use a condenwer enlarger.

Kodak is the only manufacturer that publishes CI values for its films, as far
as I know. Perhaps there are a couple of others who do the same but I am not
familiar with them.

If you are interested there is a book "Beyond the Zone System" by Phil Davis
that goes into the subject at great depth. It is basically applied
sensitometry which is a good defintion of the Zone system.

Mr. Scarpitti's postings are considered to be garbage by the majority of the
posters in this newsgroup. Once in a while he says something worthwhile but it
is lost in the noise of his hatred for the Zone System. I suggest you take
them with a grain of salt until you have read a few more of his posts and then
you can make your own opinion.

Best regards

Bert

In The Trenches wrote:
Thanks, a few more questions though, how is the CI derived?. What do I use
to measure it? What does the number actually mean? Is there a chart
somewhere? I've got a condenser head on my MX45 and I've just been
developing as per their instructions on the box of film, so probably, I've
been doing everything at .57. Could I correct this by reprinting using my
vari contrast filters or will that make the problem worse? Thx.


"Michael Scarpitti" wrote in message
om...

"In The Trenches" wrote in message

news:BqMsc.588797$Ig.273737@pd7tw2no...

Can someone explain to me contrast indexes. Are they published for each

B&W

film, or do I have to calculate it by myself? TIA.

'Contrast Index' (CI) is a measurement of contrast of B&W film, and
varies with the degree of development. Kodak's times given in their
instructions usually produce a CI of about 0.57, which is about
optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser
enlargers, and is obtained by reducing the developing times given by
about 25%.

You don't calculate this number or use it at all. It is simply for
measurement purposes.

In The Trenches wrote:

One more thing. If I'm reducing dev times should I also be rating my film
down too. i.e. 400-320 and 125 down to 100? Or what happens in this
instance.

No, you reduce the times to reduce the contrast. If you should lower the film
speed you have just undone what you did by reducing the developing times.

CI is a measurement of contrast, similar to gamma but the measurements are
taken differently and the two values are not exactly interchangeable.

I will check to see if I can find the definition of CI. Basically it is the
slope of a line drawn in the H&D curve between two specified points.

OK, here it is, taken from "Beyond the Zone System" p29 and p30. The
definition of CI is the slope of a line drawn from a point in the curve at
0.1 units above base+fog to a point that is 2.0 units away from the first
point. You may have to see the graphic to understand this.

Bert


"Michael Scarpitti" wrote in message
om...

"In The Trenches" wrote in message

news:BqMsc.588797$Ig.273737@pd7tw2no...

Can someone explain to me contrast indexes. Are they published for each

B&W

film, or do I have to calculate it by myself? TIA.

'Contrast Index' (CI) is a measurement of contrast of B&W film, and
varies with the degree of development. Kodak's times given in their
instructions usually produce a CI of about 0.57, which is about
optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser
enlargers, and is obtained by reducing the developing times given by
about 25%.

You don't calculate this number or use it at all. It is simply for
measurement purposes.

I've some some more info at http://www.infodotinc.com/photographyadv/19.htm
They've even posted a Contrast Index chart. I'm still grappling with what
all this means in practical terms to my darkroom work but I think it's worth
knowing. I have a darkroom and contrast is a big part of developing so might
as well know it all or as much as possible. Do people who develop their own
film regularly consult contrast indices? Or are they consider something to
know about and forget.?



"BertS" wrote in message
. net...
In The Trenches wrote:

One more thing. If I'm reducing dev times should I also be rating my
film
down too. i.e. 400-320 and 125 down to 100? Or what happens in this
instance.

No, you reduce the times to reduce the contrast. If you should lower the
film
speed you have just undone what you did by reducing the developing times.

CI is a measurement of contrast, similar to gamma but the measurements are
taken differently and the two values are not exactly interchangeable.

I will check to see if I can find the definition of CI. Basically it is
the
slope of a line drawn in the H&D curve between two specified points.

OK, here it is, taken from "Beyond the Zone System" p29 and p30. The
definition of CI is the slope of a line drawn from a point in the curve at
0.1 units above base+fog to a point that is 2.0 units away from the first
point. You may have to see the graphic to understand this.

Bert


"Michael Scarpitti" wrote in message
om...

"In The Trenches" wrote in message

news:BqMsc.588797$Ig.273737@pd7tw2no...

Can someone explain to me contrast indexes. Are they published for each

B&W

film, or do I have to calculate it by myself? TIA.

'Contrast Index' (CI) is a measurement of contrast of B&W film, and
varies with the degree of development. Kodak's times given in their
instructions usually produce a CI of about 0.57, which is about
optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser
enlargers, and is obtained by reducing the developing times given by
about 25%.

You don't calculate this number or use it at all. It is simply for
measurement purposes.

Excellent post. Clearly I need to do some more reading. But I think you've
provided a starting point and historical perspective as well. In general
terms should I be manipulating the contrast on the negative or is it better
to do it post development using the enlarger?


"Richard Knoppow" wrote in message
ink.net...

"In The Trenches" wrote in message
news:BqMsc.588797$Ig.273737@pd7tw2no...
Can someone explain to me contrast indexes. Are they
published for each B&W
film, or do I have to calculate it by myself? TIA.

Contrast Index, or CI, is a method of specifying contrast
devised by Kodak to be applicable to a large variety of
materials.
Contrast is the slope of the characteristic curve. This
curve shows the relationship between exposure and density.
It varies with development: the more the film is developed
the steeper the curve will be and the higher the contrast.
At one time contrast was defined as the slop of this curve
at a point on the straight line portion. Actual film curves
vary in slope from low to high densities. The difference in
slope is determined by the way the emulsion is made and
represents the distribution of sensitivity of the silver
halide particals which make up the emulsion. The shape of
the curve is decided on by the manufacturer to yield a
certain tonal rendition. The curve has three parts: the toe,
the straight line, and the shoulder. The toe is the low
density part. In general this has a lower contrast than the
straight line portion. The shadows are recoreded in this
region. The straight line portion represents most of the
film's recording range. It records differences in brightness
as a linear change in density. This is the region used for
most of the brightness information in the photo. The
shoulder is the high exposure- high density part of the
curve. Again, the contrast is lower than in the straight
line portion. Most modern films have such a long range of
densities that they never reach the shoulder in normal use
but many older films did resulting in "blocked" highlights.
Until about the 1940's contrast was usually measured as
"gamma" Gamma is the contrast of the straight line portion
of the curve. However, most films record part of the image
in the toe region and many films have relatively long toe
sections. Some, like Tri-X sheet film and the discontinued
Plus-X sheet film, have no straight line portion, the
contrast increasing with density at all values. For films
like this measuring contrast as gamma can be very misleading
about the printing quality of the negative. For instance, a
long toe film, like Tri-X Pan sheet film, when compared to a
short toe film, like T-Max 400, will have much lower
highlight density when developed to the same gamma. One
solution to this was the development of Average Contrast, or
bar-G (i.e., the letter with a dash over it to indicate its
an averaged value). Average contrast is, essentially, a
straight line drawn from the minimum density to some maximum
density. For the ISO method of measuring film speed bar-G is
defined indirectly by the specified exposure range and
expected resulting density range. For this method the range
of exposure is log 1.30. The film is developed so that the
density resulting from the maximum exposure is log 0.9 above
base density plus fog. The speed is measured between this
point and a minimum of logd 0.1 above base density and fog
for a density range of log 0.8.
Contrast Index is similar to bar-G but it specifies a
minimum density of log 0.2 above base and fog and is the
slope measured from this point to a high density point
corresponding to a range of LogE 2.0 from the exposure
needed to produce the low density reference. This is a more
realistic range of exposure in normal photography and the
reference point of logD 0.2 above base and fog is far enough
up the toe to eliminate problems with very low contrast toe
regions (i.e., no shadow detail). Contrast Index is
applicable to all sorts of film, including special purpose
films, like high contrast copy or graphic arts films.
While CI can be determined from a film curve it is easier
to measure using a graphic overlay designed by Kodak Labs.
These used to be available from Kodak but have probably been
discontinued or, perhaps, are available from Silver Pixel
Press.
CI was originally described in a paper published by the
Kodak Research Labs.

"Contrast Index" C.N.Nelson and J.A.C.Yule, _Photographic
Science and Engineering_ V. 10, No. 1, January-Febuary 1966

It is also described in detail in several books on
photographic science.

I should clarify that two negatives on different films
developed to the same contrast index will not look the same.
The curve of the film has a strong influence on tonal
rendition, however, at the same CI both negatives will print
on the same grade of paper and yield reasonably good prints.
Where they are developed to the same _gamma_ they will not
print on the same paper grade and the long toe film may have
inadequate shadow detail on any paper.

Kodak uses CI for its contrast specifications. Agfa seems to
still use gamma, and Ilford and others use bar-G.


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

In The Trenches wrote:
In general
terms should I be manipulating the contrast on the negative or is it better
to do it post development using the enlarger?


It depends on exactly what you're doing, and what type of equipment that
you're using. If you're using large format (4x5" or bigger), then most
people very the development for each sheet of film. You want to give
the exposure and development that will best convey the qualities that
you want in your print. Usually this means exposing and developing your
negatives with grade 2 paper in mind. That way you give yourself the
greatest range for manipulation of the print using different printing
contrasts.

With roll film (120mm or 35mm), it's difficult to give each frame
different development. People who vary development with these films
either have multiple cameras/film backs, or they try and shoot whole
rolls of film under similar conditions. Another consideration is that
developing a film longer gives bigger grain. In large format this is
often not a problem, but it can be with the smaller formats. Often times
roll film users aim to print on grade 3 paper. Doing so will lead to
shorter film development, which minimizes grain size.

-Peter De Smidt