These are some instructions for a class, using a copy camera and
high contrast ortho film, but the same basic principles apply when
using an enlarger and other films.
Film, Halftones and Color Separations
Density range, from minimum to maximum, gives the amount of
contrast. The density range of film or a print is measured
precisely with a densitometer, or can be estimated by comparison
with a standard gray scale. Film density is a measure of how much
light is transmitted through it, while print density is a measure
of the reflected light.
0 density is clear, and transmits close to 100% of the light
0.12 medium light, 75%
0.30 middle tone,50%
0.60 medium dark, 25%
2.0 black, 1%
Reflection density (of a specific darkroom gray
scale, but gives the general idea):
white = 0 density
step 1 = 0.15
step 2 = 0.30
step 3 = 0.45
and so on, each step being 0.15 more than the one before, up
black = 1.65
A well printed original photograph with good blacks and whites and
gray tones may have a density range of about 1.70 to 1.80. A mushy
gray photograph with no real blacks or whites might have a range of
around 1.20. Halftones give a range of about 1.10 to 1.40.
Any time something is copied, as with a photograph to a large
negative, or a negative to a positive, contrast is increased;
intermediate tones are lost.
Ortho film (line film such as Kodak EL) is sensitive to blue and
green light, and is red blind. It may be used with red safelights.
Red objects photographed with ortho film will appear black.
Line film has a very steep density-log exposure curve; it is very
contrasty, rendering an image in black and clear areas with little
or no gray tones.
High contrast is desirable for making negatives for plates, but for
blueprints, gum prints and so on, some intermediate gray tones are
needed. To lower contrast, different developers and dilutions may
be used, such as Dektol diluted 1:1 to 1:8 or more with water.
Other means of lowering contrast are: Still development, placing
the film in enough developer so it is completely immersed and then
letting it sit still without agitation for the entire development
time; Exposing ortho film through a yellow filter and with a
magenta halftone screen; or Making a bump exposure by exposing the
film only (without screen or copy) for 1% to10% of the total
Keep halftone screens clean. DO NOT handle them with wet hands.
(F.Y.I. in 1995 the small screens cost about $20; the large ones,
Use a screen larger than the piece of film so the vacuum in the
camera back holds everything in place. Use tape if screen is about
the same size as film (to keep film and screen from falling while
in the camera).
Halftone exposures consist of the main exposure time and two
optional exposures, the flash and highlight (or bump). Some
channels are set up on the camera which calculate these exposures
for you after you enter the minimum and maximum density of your
copy, and f-stop and magnification you are using. Density of the
copy may be obtained from densitometer readings or from estimating
by comparison with a gray scale. ( If all else fails, try about 0.2
minimum and 1.6 maximum). See the copy camera manual for
The main halftone exposure is made with the main lights, with the
screen in contact with the film. The flash exposure is made, also
with the screen in place, by green lights inside the camera. A
highlight exposure is made with the film and copy still in the same
place but the screen removed, and uses the main lights.On our copy
camera: Tm= main; Tf = flash; Th = highlight.
To calculate the main halftone exposure, make test strips or use a
camera channel. Halftone exposures are usually much longer than
The flash exposure adds dots to shadow areas. It brings out shadow
detail, lowers overall contrast, and compresses density range. On a
test halftone exposure, using the green flash lamps and halftone
screen, but no copy, the exposure time that gives dots slightly
larger than the shadow area dots (shadow is approx. 5% dot pattern)
is the flash exposure.
The highlight exposure increases highlight contrast, and drops out
highlight dots. It is usually about 5% or 10% of the main exposure.
It is made with the film in place on the camera back, the copy in
place on the stand, but the screen removed.
When exposing film in an enlarger or copy camera, use a gray scale
if possible. (for example: Cameraman's sensitivity guide from
Stouffer Graphic Arts Equipment Co, South Bend IN 44617, which
comes with detailed instructions.) The gray scale is a useful guide
When developing the film, it is important to wet film evenly right
away, to avoid streaks and spots later. Place film face down in
developer and then quickly turn it over and rock tray so film is
evenly coated with developer. A flat bottomed tray works best.
The usual development time, for high contrast film in A & B
developer, is 2 minutes. The developer has a range of about 1 - 4
minutes. Less than 1 min. can give uneven streaked negatives, more
than 4 min. can result in fogging and dark spots.
A good halftone negative should have shadow areas ranging from
almost all clear, 10% dots to 20 or 30%. This prints black with
tiny white dots. The highlights on the negative should be almost
black, 90% dots. This prints white with tiny black pinpoints. The
midtones should range smoothly from about 35 to 70% dot, with
around 50% dots making a checkerboard pattern.
Duotones and Two Color prints:
A duotone is made from a principal negative which favors the shadow
details, and a second negative from the same copy, which contains
the highlight detail. The position of the screen for the first
negative is 30 degrees from the position of the second, usually one
screen at 45 degrees and one at 75 degrees, to prevent unwanted
moire patterns. The plate made from the principal negative is then
printed in a darker shade and the second in a lighter color, giving
a rich print with a better density range than what could be
accomplished with one halftone.
Two-color images may be made from two negatives of the same copy,
not necessarily highlight and shadow, and the plates printed in a
strong color and a tint. A plate made from a halftone and a plate
made from a line shot of the same original may also be printed
together in two colors, or combinations of halftones and line shots
of varying exposures printed in or out of register in different
colors may be used for interesting effects.
Xerox or other copy machine or computer separations are quick and easy.
On a color photocopier, make some sort of registration marks on your original,
and then print out a yellow copy, magenta copy, cyan copy and full color
or black copy. It is best to then make film negatives from the photocopy color separations. It might be possible to use the color paper photocopy separations as is, but various gum, plate, blueprint, etc. emulsions are sensitive to different colors, and while, say, a yellow photocopy might work as a negative in one case, it might act like a perfectly clear blank negative in another. As always, experiment with some small test prints first. With a computer, in an image-manipulation program such as Photoshop, there are file or printing menu choices for color separations. In Photoshop, choose the CMYK image mode, then print each channel separately (in black and white). See the instruction manuals that come with the computer programs for more detail. Computer or photocopier prints
already contain a screen pattern, so making halftones is not necessary. Line
negatives or plates can be made from the printout separations. Printouts or photocopies can be made on the regular thin paper you find in photocopiers and computer printers, or on clear plastic (the kind used for transparencies, viewgraphs, overheads). It is best to use the type of clear plastic recommended by the printer manufacturer; sometimes the inks or pigments run or don't stick if it is the wrong kind. Paper works just as well as plastic for negatives, but needs longer exposure times, roughly twice as long; and in some cases, usually with shorter exposures or paler, thinner originals, the paper
texture may show up in blank areas.
Separations may be made in a 35mm or 2-1/4 camera on a tripod, with
general purpose black and white pan film. Make separate exposures
of the same subject through blue, green, and red filters. Put
registration marks on the copy and use a gray scale in each
exposure. Print the resulting negatives on 5 inch x 7 inch; or 8
inch x10 inch paper, so that a chosen step on the gray scale is the
same in all prints, and rephotograph these in the copy camera with
halftone screens. Make the separations on the same day with the
same settings, otherwise there may be registration problems.
Separations may be made with filters in the copy camera. Use ortho
film for the blue and green separations, and pan film for the red
separation. The halftone screen must be photographed at different
angles for the different colors to avoid unwanted moire patterns.
The standard angles are: 45 degrees for black (no filter, or yellow
filter), 75 degrees for magenta (green filter), 90 degrees for
yellow (blue filter), 105 degrees for cyan (red filter). Make all
the color separations and halftones the same day, using the same
settings, to avoid variations in negative size which make printing
The plates are printed in this order: yellow, magenta, cyan, black.
The screen dot pattern is most noticeable at 90 degrees, so it is
printed yellow, which is harder to see.
Intermediate shades may be added to an image with per cent screens.
These are exposed at the same time as the flat, in contact with the
plate, 30 degrees from halftone negatives.
Pocket Pal A Graphic Arts Production
Handbook, International Paper Company, International Paper
Plaza, 77 West 45th Street, New York, NY 10036. Includes halftones
and registration, along with type, inks, paper, printing processes,
etc. My copy, the 12th edition, was published in 1981 but I think
there are newer editions out.