[Tig] instinctive contrast

[S. T. Nottingham III]

I am sure most good engineers could give you a technical explanation of this
phenomena, and for someone on the level of Dave Tosh (whom I really admire
and respect), it shouldn't really be difficult to articulate the reasons. My
experience comes from over 25 years in the chair, and also a dose of several
years on the other side in a film laboratory. Since my background is a
technical one of film and television, and not one of engineering, please
forgive me if I gloss over the electronic theory, and dwell on the
metaphysical.

The characteristic you observe is the primary reason I dislike the practice
of shooting on color stock when finish is intended to be black and white.
Quite simply, the gamma response of color stock is not even close to that of
Panchromatic Black and White stock. Of course unless you know what to look
for, or happen to be able to see the results projected side-by-side, the
casual observer might not even notice the difference in gamma when contrast
is restored in Telecine. The experienced operator understands that true
black and white images neither posses true black or white, but consists only
of grey scale. A classic film such as Casablanca is a good example. During
most of the film, blacks rarely go below 3 - 7 ire (or  above 10 ire in
NTSC), and the whites rarely clip except for specular highlights. Black and
White transferred any other way just does not look natural. Properly exposed
and processed  b&w stock has a gamma curve of somewhere around .3 to .4.
Gamma is defined as the slope of the straight-line portion of the D log E
curve when measured on a densitometer.

A slope of 1.0 is a curve that produces equal increments of density for
equal increments of exposure. Contrast index is the difference between the
toe and shoulder of the straight line portion. D min is the clearest (no
density) frequently referred to as base + fog, and D max is the densest. A
properly exposed and processed black and white film rendering of a normal
contrast scene has a contrast index of about 1.8. Black and white stocks are
frequently referred to as having an extended shoulder which means they can
be greatly overexposed and still yield usable images, although the contrast
index is greatly affected. It was this very phenomena that Ansel Adams
exploited in his development of the Zone System.

Color stock is another animal. Composite color stocks are in reality three
separate layers of black and white silver images that are exposed through
three separation filters built into the emulsion. These three layers
represent the Red, Green and Blue records of the original scene. During
processing, the silver is bleached and removed, leaving three layers of dye
representing the complement of the original RGB. That would, of course, be
yellow, magenta and cyan dye. Since there is no "black printer", contrast
and luminance are derived by a combination of the three. Positive prints
(for projection and made from OCN) can have a contrast index as high as 2.8
to 3.2. Three-strip Technicolor or Kodachrome can have contrast indexes of
3.8 to 4.1. Gamma frequently falls between .6 and .8 - still less than 1.0,
but considerably higher than black and white stocks.

Chromatic values consist of three components: Saturation, Hue, and
Luminance. All three interact to make up a color stock's contrast, with
colors like Red and Yellow normally having high luminance values, and others
such as Blue and Cyan normally having much lower luminance contents. This
can be seen when RGB outputs from Telecine are viewed separately on a black
and white monitor. The Blue record normally has the lowest contrast, and the
Red record has the highest. Green is considered "normal" which is why the
original Rank Encoders derived luminance from the Green channel. This
phenomena can be see in the Lab as well as in various TV systems - it is the
nature of color as it occurs in the environment. Of course, there are always
exceptions. Blue can have a high luminance content in night-club scenes with
strong color lighting, but the in the normal scene, the Blue record has
fairly low gamma.

In Telecine, a normal scene is set up with blacks at zero, and whites well
into clip. This is especially true of commercials because clients never
think there is too much contrast or chroma. Heavy chroma content adds
considerably to the luminance content of the various colors, but does not
add to the basic "black and white" value of the scene. Where it is true that
modern-day color correctors such as the DaVinci 2K derive luminance using a
complex formula, and also allow the operator to control it separately, when
chroma is eliminated, the four channels become one of the same value. Any
luminance added to the original scene due to color saturation or hue is now
removed - leaving nothing but the luminance scale. Since contrast is the
difference between the lightest and darkest values in the scene, removing
chroma reduces contrast. Bright colors, such as Red become darker grey
values, and dark colors, such as Blue become brighter. This effect can be
seen using a switcher to dissolve between 75% color bars, and its mono
component.

When transferring black and white films for colorization, care must be taken
by the operator to build sufficient contrast into the luminance values
because of the basic differences between color and black and white stocks.
Also, a color added later cannot remember the "chromatic" luminance values
from the original scene, so this must be taken into account in order for the
final result to look like it was shot on color stock. By not adding contrast
in Telecine, the same scene when viewed later with chroma added becomes very
dark and muddy. A lot of the black & white classics from the 30s and 40s
were shot on Orthochromatic stocks - which were only blue sensitive. Even
though the studios frequently over developed these stocks to increase
contrast and increase film speed, it did not entirely overcome the flat
nature of Orthochromatic films. They also did not represent the higher
luminance values of warm colors.

The effect of Chroma and Hue on a scene's contrast is really powerful, and
is automatically taken into consideration by a good colorist whether he is
aware of it or not. After all, luminance is only one third of the picture.

Tom Nottingham
Retired Colorist



 -----Original Message-----
From: 	tig-bounces at tig.colorist.org [mailto:tig-bounces at tig.colorist.org]
On Behalf Of Rob Lingelbach
Sent:	Saturday, March 17, 2007 8:39 AM
To:	Telecine Internet Group
Subject:	[Tig] instinctive contrast

Thanks to Cintel International for support in 2007.
http://www.colorist.org/wiki3
====



Something I think all experienced colorists know instinctively
deserves a
scientific explanation, if someone here would like to attempt one.
Take a color image, make it look the way you think it should, and then
extract all the color, making it pure black and white.  Unless you're
specifically looking for low contrast, you will now need to dial in
significant amounts of contrast (drop the blacks, raise the whites).
(the only other exception I can think of to this formula is if the
original image was extraordinarily high in contrast when it was in
color.)