[pure-silver] Re: Kodak vs Ilford
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sat, 22 Mar 2008 16:29:04 -0700
----- Original Message -----
From: "mail1" <mail1@xxxxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, March 21, 2008 10:26 PM
Subject: [pure-silver] Re: Kodak vs Ilford
Richard, Your comments on film curves verses film types is
excellent.
Thank you:-)
I didn't talk about the shoulder because most modern
films don't really have a shoulder until they reach
impractical densities. Most B&W films have an enormous
overexposure latitude, six to 10 stops in many cases. This
is not the _range_ of the film but the amount the exposure
can be increased from the ISO exposure and still have good
tone rendition.
I suppose I should define a shoulder. Just as the film
has a low contrast area for very low exposures (the toe) it
also looses contrast for very high exposures. At some point
the film reaches its maximum density and will just go no
further. The theory books will show an area of reversal for
extremely high exposures but modern films do not seem to
ever reach this point. As the highest densities are reached
the contrast is reduced and this part of the curve is called
the shoulder.
A shoulder can also be produced by the developer. Some
developers just are not active enough to develop the highest
possible densities on the film. If you look at the data
sheets for Kodak films which show development in the
Versamat automatic processor you will find the higher
densities shoulder off at noticably lower densities than
when developed in something like T-Max or even D-76.
It is possible to generate an artificial shoulder by
using what is called a "compensating" developer. These are
highly diluted or otherwise designed to have too low an
activity to reach the highest densities. Also, development
without agitation will result in local exhaustion. In the
dense areas the developer reaction products build up slowing
down development. In normal processing the agitation removes
these reaction products and allows fresh developer to
penetrate the emulsion. Not all developers are equally
sucessful in stagnant development because some reaction
products actually accelerate development rather than
restraining it. A developer containing Metol as the sole
development agent will exhibit compensation and also edge
effects because the reaction products of Metol are
restrainers. Rodinal, which is related to Metol behaves the
same way. The reaction products of hydroquinone, OTOH, are
accelerators and a hydroquinone developer will not show the
same sort of effects from low agitation as one having only
metol in it.
Printing negatives with very high density highlights is
a problem. If the image is not too complex the exposure of
different areas can be adjusted by burning and dodging. To
do this well (so that its not obvious on the final image) is
not too easy and requires a lot of practice.
Local contrast can also be reduced by the use of
contrast masks. These can be made with a contrast higher
than the negative so that they result in a non-linear final
curve. Contrast masks used to be common in color printing
especially with Cibachrome. They are not too difficult to do
for B&W. Similar effects can be gotten from electronic image
editing programs like Photoshop but many are not aware that
they can be generated photographically albeit with more
work.
A last word on film curves: I think the best explanation
of their effect is in the older Kodak film databooks. I have
no idea where to find these other than camera swap meets or
dealers specializing in this kind of thing. Its possible
some library systems may have them. The edition is not
important because the same text was reprinted with the film
data sheets tipped in. Its the text you want so having
obsolete data sheets is of no consequence.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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