Re: Best CI for process

From: Richard Knoppow ^lt;[email protected]>
Date: 10/02/05-04:00:04 PM Z
Message-id: <001101c5c79c$a7d7a3c0$75f85142@VALUED20606295>

----- Original Message -----
From: "Sandy King" <sanking@clemson.edu>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Sunday, October 02, 2005 12:57 PM
Subject: Re: Best CI for process

>I would put it this way. Rather than saying that different
>processes *require* a DR independent of CI, I would suggest
>that for subjective reasons some may chose to use a DR
>independent of CI. It is obvious that the use of different
>DRs, when the CI remains the same, can result in images of
>different look because of the way the film curve interacts
>with the paper curve. This is obviously a very complex and
>quite subjective issue that does not get a lot of
>attention. But it is clearly an especially important
>concern with processes like platinum and pallaidum, which
>have by nature very long toes and shoulder compression.
>
> By contrast, the issue is basically moot with a process
> such as carbon that is almost perfectly straight line.
>
> Sandy
>
>
  I think the key here is distinuishing between contrast,
however stated, and curve shape. Average contrast, or G-bar,
and in particular the variation of it known as Contrast
Index are methods of measuring contrast of negatives such
that the density range is the same regardless of the curve
shape. Note that the tone rendition will not be the same for
prints made from negative materials having different curve
shapes. This will be true for any printing method, silver or
alternative included.
  There are thee methods of measuring contrast: 1, gamma.
Gamma is a measure of the slope of the exposure vs: density
curve at a straight line portion; 2, Average Contrast, or
G-Bar, this is a measure of the average slope of a curve
between two arbitary densities;3, Contrast Index, this is a
method devised at Kodak which is similar to G-bar except it
measures the average slope between two specified density
points separated by a log exposure interval of 2.0 This is a
suitable range for conventional printing materials. CI also
specifies a starting point density that is far enough up the
toe slope to have usable contrast.
   Some alternative printing methods tend to compress
shadows. The reasons for this are not important here, only
the fact that it occurs. This tends to make normal negatives
look flat. However, simply increasing the contrast of the
negative may still not result in acceptable tone rendition
on the print because the compression of high exposure areas
still takes place. This is the equivalent of a negative with
a very long toe. Probably the best materials to use for
making such negatives are very short toe films like T-Max
used with lenses that have very low flare. Flare produces an
effect similar to a long toe on film (at least in B&W).
  An advantage of making digital negatives is that the
effective curve shape of the negative can be adjusted at
will to be suitable for the printing method. If non digital
methods are used a somewhat similar effect can be gotten by
making masks for the negatives, but these are problematical
for contact printing methods.
   The contrast of a printing material is the range of
exposure needed to go from paper white (or Dmin) to maximum
density. If this range is large then a negative having high
Dmax will be needed to get the range. However, this may not
be enough to get good tone rendition for the reasons
mentioned above.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com 
Received on Sun Oct 2 16:00:44 2005

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