Before the halftone process was invented, allegedly by Henry Fox Talbot in the 1830s, illustrations all had to be printed via engraving or some such laborious technique. The halftone process, as it evolved from its shaky beiginings in 1873 (see Wikipedia’s article) takes a photo (and of course the invention of photography would clearly be a necessary precondition) and converts it into a series of regular lines of dots of varying sizes. imagesBigger dots would create the shadows, smaller ones the highlights. The exaggerated example at the left shows this in extreme. Retreat across the room and look at it and you’ll see a perfectly recognizable portrait. The finer the screen — the more lines of dots per inch — the greater the detail that can be reproduced. However printing technology and paper surface characteristics impose an upper limit to the screen value. Most regular book work is done at 133 lines per inch, while an art book on smooth coated paper will probably go to 150 lpi. Newspapers tend to screen at 85 lpi. The screen would be imposed by the simple process of mounting a transparent ruled grid in front of the photograph so that the process camera would only “see” a series of dots when it shot the art. (See also Dot gain.)

In the olden days publishers’ production staff used to do a lot more things for themselves, things which nowadays we have learned to slough off onto our suppliers (or software). Using a screen finder is one of these things. Authors often submit printed photos as artwork for their book. As I explained in my post Moiré, “an already printed photo will carry its original screen, and when it gets rescreened for your publication there’s often (usually/always?) a conflict between the two screen angles which sets up a pattern of darker and lighter areas regularly spread across the picture.” This patterning is called moiré and one should strive to avoid it. If you know the screen value of the original piece, you can adjust the screening you now have to apply to it to minimize the conflict between the two screens — just why we thought we had to provide the printer with this information, rather than trusting him to discover it, may amount to little more than that: trust. There was a time when workers kept their heads down and did what they were told: no less and importantly no more. So you’d tell them.

img_0446The screen finder shown above, is printed on a transparent plastic sheet. It consists of a series of thin lines, radiating from some distant vanishing point. When you lay this over a printed halftone and jiggle it around to pick up the screen angle it will form the sort of cross found in a moiré pattern, and the arm of that cross will be found to be pointing to the number of the screen value printed along the bottom of the oval. Quite ingenious. These screen finders, cheap to produce, were often given out by suppliers as gifts to their customers. This one came from Arcata Graphics Prepress.

 

Advertisements