Re: Best CI for process

The more you guys talk about this, the less I think I understand.

If CI is arrived at by choosing a starting point density, counting off
2.0 log exposure units from there to find the end point density, and
calculating the average slope of the line between those two points, and
if log density and log exposure are related in a lawful way, which I'm
told is so, then negatives with exposures that fall beyond the end of
the portion of the curve for which the slope is calculated, like the
three negatives Sandy listed earlier, will have to have the same CI
almost by definition, simply as an artifact of the arbitrary truncation
of the curve. It looks to me the CI is hitting an artificial ceiling,
and that in order to get a meaningful CI for these negatives, you'd need
to calculate the slope of a curve section that extends into a longer
exposure range.

If, as Richard says, the length of the portion of the curve for which
the slope is calculated to arrive at CI was set at 2.0 because that
range was deemed adequate to cover conventional printing materials,
does it make sense to apply it to platinum, which has a longer range?

I admit I am rather out of my depth here, as (1) I know absolutely
nothing about platinum, (2) I'm one of those people who work intuitively
by the seat of my pants; I've never used or owned a densitometer or even
a light meter, but I always seem to manage to get what I'm after
regardless, and (3) I use a print medium that has a fairly short
contrast range and prints well from a less dense negative, so I've had
no need to try to extend the contrast range of negatives. But I do know
quite a lot about statistical artifacts (if you want to hear a real rant
ask me about standardized test scores reported as "grade equivalents"!)
and when I see a variable hit something that looks like an artificial
ceiling, I sit up and take notice.

Or am I missing something obvious?
Katharine

Richard Knoppow wrote:
>
> ----- 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 20:49:36 2005