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.

Notes on Film, Halftones and Color Separations

Film density:

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.

Transmission density:

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 to:
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 exposure time.

Making Halftones:

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, $200+)

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 details.

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 line exposures.

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.

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.

Color Separations:

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 difficult.

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.

Screen tints:

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.

More Information

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.

and there's a little bit more information on my high contrast films and
registration pages.

* step guides (gray scales for use in the copy camera) can be obtained from Stouffer graphic Arts http://www.stouffer.net/Productlist.htm or from many printers' supply companies.

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last update 8-3-02