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Process photography is the term used for the photographic techniques employed in the graphic arts process. Some platemaking processes require negatives and some positives. Some require correct reading on the plate, and others, reverse reading. The negatives or positives can be line, continuous tone, or halftone.

Any picture or scene consists of a number of different tones or gradations of tones known as continuous lone. In a photographic print, these are represented by varying amounts of silver in the print. The more silver, the darker the tone, and vice versa. In letterpress or lithography, these tones can not be represented by varying amounts of ink with a single impression on the press. In order to produce the tones necessary to reproduce a picture, these processes must use the halftone principle. This is an optical illusion in which the tones are represented by solid dots that all have equal spacing and ink density but vary in area These dots are too small to be seen individually at the usual reading distance, but it is possible lo see them by close examination of a halftone newspaper illustration or with a magnifying glass on finer screen rulings. There are several nonscreen printing processes that can print varying densities of ink and use continuous tone negatives or positives. but they are difficult to work and not widely used. These include conventional gravure. screenless lithography, and collotype or photogelatin. Conventional gravure and screenless lithography, and collotype or photo gelatin. Conventional gravure and screenless lithography are described later in this chapter. In collotype, the image consists of reticulated gelatin which prints ink directly in proportion to the amount of exposure received through a continuous tone negative.

Process photography is completed on large cameras with suspension systems to eliminate the effects of vibration on the images, since long exposures are used in comparison with amateur or commercial photography cameras can be either horizontal or vertical. Horizontal types have bed or overhead suspension. Most process cameras have a copy board, lens, and bellows in the camera room, and a camera back built into the darkroom wall. Enlargers are also used, especially for making colour separations for colour reproduction.

Lenses for process work are coated to reduce flare and are usually of symmetrical design to eliminate distortion in the images. Because of the requirements for high resolution and minimum aberration, process lenses have fairly small maximum apertures -from F/8 to F/11. Focal lengths range from 8 in for wide angle lenses for 20-in. cameras to as long as 48 in for a 40-in. camera. Some cameras use prisms for reversing the image laterally, as is required for photoengraving. cameras use prisms for reversing the image laterally, as is required for photoengraving.

The films used consist of special high contract, orthochroirvatic emulsions of silver halides in gelatin on a stable film base for line and halftone photography, continuous tone orthochromatic film for gravure, and continuous tone panchromatic film four colour separations and masks. Special developers are used for high contrast. An important and significant trend in recent years has been the increased use of automatic processing machines which are capable of developing. fixing, washing, and drying film in less time and with less variation than hand processing.

Line photography

Line copy consists of solids, line drawings, and text matter. In photography the copy is placed on the copyboard of the camera, illuminated by high intensity lights, and focused to the correct size on a ground glass in the back of the camera. The film is placed on the vacuum back of the camera which is put in place of lathe ground glass. The aperture is set on the lens, and the exposure is made through a solenoid shutter operated manually using a stopwatch, or automatically by a timer or a light integrating meter. The film is then developed, fixed, and washed either in trays or in an automatic processor. This operation produces a high contrast negative which is reverse reading on the emulsion side.