> Ok, I understand the MTF curves and the spectral sensitivity
> curves, but
> what exactly am I looking at on the characteristic curves and
> contrast
> index curves for 100TMX? I can see for instance from the CI
> curve that
> if I process the film longer, the CI goes up, but how is the
> CI defined?
I personally don't spend too much time worrying about CI curves
as my developer of choice (DD-X) works a bit differently.
However, the explanation is this: A CI curve is essentially
indicating a ratio of some form of the maximum density of the
negative. Look at the following link:
http://www.kodak.com/global/en/professional/support/techPubs/f4016/f009_0450ac.gif
You can see from this that development time of most developers
will adjust the contrast of the negative. The more you develop
it, the greater the contrast. TMX100 in HC100B for 6 minutes
will produce a soft negative that is good for preserving
highlights. (expose for lows, develop for highs). Processing the
same combination for 14 minutes will increase the contrast to
the maximum possible with the emulsion.
This is only part of the story, though. This gives you the
development times to handle maximum and minumum contrast ratios
with that particular film/developer combination. But it does
not indicate anything about the shape of the response curves.
http://www.kodak.com/global/en/professional/support/techPubs/f4016/f009_0439ac.gif
The above graph shows that TMX100/Tmax Developer does have a few
nuances that bug many people. With the 12-minute curve, you see
a Zone IV bump but a Zone VI dip followed by an out-of-control
lifting of the highlights that will cause blocking. 7-minute
developement brings most of these peaks and dips into alignment
and creates a smooth straight-line section that goes right up to
the maximum density at that development time. TMX has no
"shoulder" to speak of. As such, TMX is a film you do not want
to over-expose. You must nail the exposures.
Now let's look at Tri-X:
http://www.kodak.com/global/en/professional/support/techPubs/f4017/f009_0492ac.gif
What you see here is a long straight-line section, a gently
curving toe (preserving Zone I-II details)and a slight shoulder
that preserves highlight details.
One more to compare. Let's look at Plus-X in D-76:
http://www.kodak.com/global/en/professional/support/techPubs/f4018/f009_0433ac.gif
The toe is similar to Tri-X, the straight-line section is even
from Zone II-VII which gives near-perfect tonal seperations and
a gradient to die for. Look at the shoulder. See how it
flattens out? You will find details in the highlights and it's
a bit more forgiving to over-exposure. You can see why this
film/developer combination is so loved. Other than grain
structure, this is among the very best film curves ever.
One last one to look at. Ilford XP2:
http://www.ilford.com/html/us_english/pdf/XP2SGB_QX.pdf
On page two of the PDF you see the characteristic curve. First
of all, note that the density of the negative is very low.
Secondly, see that the response curve goes right from the toe to
the shoulder. The greater the exposure (more light), the more it
gets compressed. There is no straight-line section so you do
lack tonal seperation and an even gradient from high to low.
Even though highlight details are preserved, there is so little
tonal seperation from Zones VII-X that the highlights lack snap.
I'm not dissing XP2, here, just pointing out how it compares to
traditional films. When scanned (it scans very well), you will
usually need to lift the highlights to counter this compression.
In the darkroom, it favors papers that have opposing response
characteristics. (Forte).
So, back to the original question. CI essentially refers to the
maximum density on the negative of the recorded highlights. The
denser the negative, the less contrast is required for the
paper. There are two schools of thought--high contrast neg/low
contrast paper or low contrast neg/high contrast paper.
Personally, I like middle of the road ones that give me some
flexibility either side during printing and stand up well to
split-grade printing and paper-flashing. If you scan the negs,
you will want to lean towards softer negs. High-density negs do
not scan well and you'll lose highlight details. As always
YMMV. It's best to pick a middle-ground than to hit the
extremes.
AG
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