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USING A COLOR ANALYZER
At the beginning and for modest printing
requirements, the ''trial and error" method of
determining exposure and filtration can be used.
However, this method is wasteful of time and
materials. The best way of producing consistently
good color prints is to use an Omega Color Analyzer,
several models of which are described in the enclosed
literature.
The analyzer is, in essence, a mini-computer.
Information is gathered by a probe placed on the
baseboard of your Omega enlarger which "reads" the
print as projected. By suitable adjustment, correct
exposure times and filtration settings can be
determined.
EXPOSURE COMPENSATION
Color papers have three emulsion layers, each
sensitive to a different primary color. The red
sensitive layer is at the top, the green sensitive layer
in the middle, and the blue sensitive layer at the
bottom. These emulsions have different sensitivities
for long and short exposure times which means that
when your exposures are either very long (over 40
seconds) or very short (under 5 seconds) you will get
color shifts. This is an effect due to reciprocity failure
which means that the colors you get on your print will
not be the same colors you would get at a "normal"
exposure time.
To avoid color shift, make your changes in exposure
by opening or closing the lens aperture (f/stop) rather
than by increasing or decreasing your exposure times.
Generally speaking, your exposure times should be
within the 10 to 30 second range for optimum results.
LIGHT ATTENUATOR
A third method of controlling exposure is to reduce
the light reaching your paper through the use of light
attenuators. These attenuators are simply slipped in
or out of the slot at the top of the C760 Lamphouse.
Each attenuator will produce a reduction in light
equal to one, two, or three f/stops.
14
FILTRATION AND EXPOSURE TIME
Because printing filters reduce the amount of light
reaching the paper, your exposures when printing with
filters will be longer than they would be without
filters. The higher the filter value, the longer the
exposure required since the higher values represent
greater densities.
For example, an increase in the density of the
magenta filter by 10 CC, requires an increase in
exposure of about 10 percent. However this increase
is not constant for all three filters:
a 10CC increase of
yellow requires an increase of only about two percent.
In actual practice, when you are working with both
yellow and magenta filters,
a simultaneous
increase
in filter densities by 10 points (i.e., 20Y to 30Y and
from 40 M to 50 M) requires a total increase in
exposure time of about ten percent. This is because
the effect of changes in yellow filtration alone are
usually negligible.
In making exposure adjustments, it is usually
preferable to adjust the lens aperture rather than
exposure time. A useful approximation is to increase
your lens aperture by about half a stop when the
magenta filtration is increased by 45 points (for
example, from 30 M to 75 M) and when there is a 35
point increase in both magenta and yellow filtration
(for example, from 10 Y to 45 Y and from 50 M to 85
M). If you reduce your filtration, then make similar
adjustments but in the opposite direction, decreasing
your lens aperture or shortening your exposure times,
thereby compensating for the increased light reaching
your paper.
EXPOSURE TIME WITH THE CS-50
AUTO EXPOSURE CONTROLLER
The adjustments outlined above can be handled
accurately and consistently by using the Omega
CS-50 Timer and Automatic Exposure Control (Cat.
No. 480-701). With the CS-50 unit, there is no need to
calculate exposure adjustments when changing
filtration, magnification, or lens aperture. The
information on changes is collected using the spot
reading probe and transmitted to the exposure control
unit. The CS-50 unit, in turn, will automatically provide
the correct exposure. It can also be used to determine
proper grade of poly-contrast paper. The unit can be
programmed for repeat exposures for multiple prints
and can be used for both color and black-and-white
printing.
Black and White Printing
NOTE: See instructions with Vari/Con Filter Module.
The C760 Dichroic and Diffusion Lamphouses supply
diffused light for using either graded or variable
contrast papers.
PRINTING BY DIFFUSED LIGHT
Because a black-and-white negative consists of
minute silver particles embedded in a gelatin base,
light passing through this material is not only
absorbed in varying degrees, depending upon the
densities of the negative, but '"'scattered"' very much
like light shining through a dense fog. Because of this
"scatter" the contrast of a projected image will
depend greatly upon the degree of diffusion of the
light passing through the negative.
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