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From: alan@sun.uucp (Alan Marr)
Newsgroups: net.graphics,net.rec.photo
Subject: Re: Solarization and Posterization
Message-ID: <2814@sun.uucp>
Date: Mon, 23-Sep-85 23:29:43 EDT
Article-I.D.: sun.2814
Posted: Mon Sep 23 23:29:43 1985
Date-Received: Thu, 26-Sep-85 06:20:36 EDT
References: <6700033@datacube.UUCP>
Reply-To: alan@sun.UUCP (Alan Marr)
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Xref: watmath net.graphics:1137 net.rec.photo:1542

Photographically speaking, there is a third term, "Sabattier
Effect" (sah-BAH-tchi-a, French), that is the correct term for
what is often called solarization.

Posterization is where the myriad tones (color or B&W) present
in a photo are reduced to a small number of tones, typically
only three or four because the photographic process is time
consuming since you must make internegatives at selected
exposure levels on high contrast film and register them
carefully when printing.  High contrast images (two tones only)
are a subset of posterized images.  Posterization is one of the
simplest image processing techniques for computers (often
called bit-slicing) and in computer graphics similar effects
are achieved with colormap manipulations.

Solarization is the process whereby film is exposed in the
camera to such massive overexposure that partial reversal of the
image occurs.  The exposure is about 1000 times normal.  It may
not work with modern films.

The sabattier effect is a darkroom manipulation that can use
paper or film.  When I did it I exposed a print normally, and
then after about 15 secs development (out of 90 secs normal) I
flashed it with about a second of room lights.  Electronic flash
can be used and is easier to calibrate.  The print develops
very quickly thereafter and in 15 seconds I very fluidly whip it
into a fresh tray of stop bath to arrest the development quickly
without getting uneven development streaks.

Partial reversal takes place, but so do Mackie lines between
highly contrasting tonal regions.  This is because the
development that occurs in dark areas generates a lot of bromide
byproduct that restrains the development.  Agitation in normal
processing distributes the bromide from the print into the
solution so with the Sabattier effect it is important that the
developer be still at the moment of flash exposure and not be
agitated afterwards.  The Mackie lines are quite beautiful,
especially where there is organized fine detail as in hair.
Stunning effects can be obtained with film by doing sequences of
Sabattier effects, building up the transformations.

I found that the best papers for Sabattier effects were high
contrast papers, especially Agfa Brovira #6.  I had very
interesting results with a box of years-old Kodabromide #5 which
would yield orange and purple colors in the re-exposure areas.
These were sensitive to light so I didn't show them much (I
fixed them minimally).  The colors were undoubtably due to
finely divided silver that was coming out, very similar to the
"print-out" papers that used to be used for contact printing.

---
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