"Monica Schulz" <monica.schulz@[EMAIL PROTECTED]
> wrote in message
news:7ec52008-4faa-4941-95b0-7e9612521257@[EMAIL PROTECTED]
ng,
I'm just begining to dive into the photographic process, so
my
practical experience is still scarse. - I try to figure out
the basics
in a theoretical way first. In case of the negative/positive
process
(both b&w and color) there is the relation****p of contrast
and gamma
and I simply can't get that into my head, no matter how much
I read
and think (maybe I read the wrong papers?!).
One thing is that negative gamma and print gamma should
combine to 1
if the contrast reproduction should match the scene (or
1,1/1,2 if you
take into account that most people prefer a slightly
exaggerated
contrast of the midrange values).
If thatīs true and the negative is developed to a kind of
normal gamma
of 0,6, print gamma must be about 1,7. In the case of b&w
contrast
must be balanced between the negative and the print if
contrast is to
be reproduced correct. So if the neg has, say, 1,3 log units
it must
be printed on a paper that is also capable of 1,3 log units.
Not more,
not less. So if you chose the paper grade according to the
neg
contrast, doesnīt that mean that you might have to chose a
grade that
leads to a gamma which doesnīt combine to 1 with the neg?
Another thing that give me headaches in the same context are
color
prints. In the case of the color reproduction chain, the
negative is
normally developed to a gamma of about 0,6 and most papers
have a
gamma of more than 2,5, so the combined gamma is 1,5. - So
how does
that work? And why are color papers not available in
different grades
to balance with the neg contrast?
You see Iīm kind of lost with those concepts and I really
hope that
someone could shed some light on them to show me whatīs
going on.
Monica Schulz
First of all "gamma" is a measure of contrast but is not
often used now. Gamma is the slope of the straight line
****tion of the film or paper characteristic but, since most
characteristics have only small straight line sections, or
sometimes none another measure is used. This is average
contrast or G-bar, the slope of a stright line drawn between
a shadow and a highlight point on the curve. Kodak goes one
step further and uses Contrast Index or CI which is the
average contrast between two specified points on the curve.
Since these are constant CI of different materials is easy
to compare.
You are right that the overall contrast of a print
should be on the order of 1 but reflection prints are
limited in the range of brightness they can reproduce so the
unity contrast is usually true only of a range of mid tones,
the shadows and highlights generally being compressed or
simply going out of the range the paper can reproduce. Since
transparencies don't rely on ambient light their range can
be greater but the target contrast is still 1. However, the
eye tends to judge "correct" reproduction of tones by the
gray tones and, for the most part ignors deep shadows and
bright highlights, so, if the mid-tone contrast is right the
eye will see the print as being right even though the
overall range of brightness that can be reproduced is
substantially less than in the original scene, or, for that
matter, less than what the negative will record.
For many scenes of relatively high contrast a low
contrast print that attempts to reproduce the full range of
tones recorded on the negative will simply look flat.
To some degree this can be compensated for by localized
printing of details, either holding dark areas back to
brighten them or printing in (burning in) highlights, but
this must be done with skill or the manipulation will be
obvious to the viewer.
For black and white work, where there is little
standardization, it is common to develop negatives to a CI
of anywhere from about 0.4 to 0.7 depending somewhat on the
kind of illumination used in printing. The printing light is
im****tant because of something called Callier effect. This
is the effect on effective density caused by the relative
diffusion or specularity of the light source. Because most
B&W films have an image made up of descrete silver
particals, which can scatter light, there is a fairly large
Callier effect. Between a completely diffuse light source,
like a color head or a cold light head, and the usual partly
condensing head, the contrast difference is about one paper
grade, the condenser head having the greater contrast. Note
that Callier effect has no effect on the one reproduction
scale other than overall contrast which can be completely
compensated for by either a change in development time of
the negative or choice of paper contrast. The method of
compensation is a matter of convenience.
Because color films, and here I include chromogenic B&W
film, the image is made up of semi-transparent dye, there is
very little Callier effect so the printing light makes
little or no difference in effective contrast. While there
is some choice in contrast of color printing materials it is
much less than for B&W. This is because the eye is much more
sensitive to variations in the contrast of color. If color
is too low or too hight in contrast the effect is quite
obvious. So, the contrast of color recording and printing
materials is much more highly standardized than for B&W. In
addition to this the three recording layers (some films have
many more) for the three basic colors must have
characteristics which match very closely if the color is to
track for all brightnesses. Anothere way of putting this is
that the neutrality of a gray scale must be constant. For
this reason also there are strict limits on the contrast
range of color materials.
Color negative materials, which are intended to printed
to make postives, have relatively low contrast and can be
controlled to some extent in processing, as long at the gray
scale tracking mentioned above remains acceptable. Color
positive materials are generally meant for direct viewing
and are much higher in contrast, generally slightly more
than 1 to compensate for flare. There ARE some transparency
materials intended for duplication or other printing which
are lower in contrast bu they are rarely encountered by
amateur photographers.
Some color materials intended for direct reversal
printing from positive transparencies offer a lower contrast
grade for use when the usual grade is too contrasty. This
was sometimes the case with Cibachrome/Ilfochrome. Another
method of reducing contrast is masking, the use of a very
low contrast negative image overlying the postive image to
reduce the contrast.
I think I am getting deeper into this than you want to
go.
You can find some elementary material on sensitometry,
the name for the study of tone reproduction from film, on
the Kodak site. Search for H-740. If you can't find it
write me privately and I will send it to you.
--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]
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