"The truth is, a halftone is nothing more than a kind of magic trick"
Bill Stephens, printer
The popularity of photography soared during the 1800s and with that grew a desire to print photographs in books and newspapers. However, the printing press was not capable of producing the continuous tone images, with infinite shades of gray, of photographs. It was not until 1881 that American Frederic Ives (1856-1937) developed the first successful halftone process.
Halftone printing involves converting continuous tone images to images made of dots of various sizes. The key to this system is that it exploits the limitations of the human eye. With the right resolution, the individual dots cannot be seen, resulting in the illusion of shades of gray, with larger dots appearing as darker shades.
The first step is to produce a negative using a process camera. The process camera has a screen between the lens and the film that contains a grid. The grid divides the image into small squares through which light passes, resulting in discrete spots. The different size spots are made as a result of different amounts of light landing on the film; more light results in bigger spots. The negative is then used to produce an engraving, often by acid etching of a metal plate, with image areas left standing proud of the surface. The resulting plates are then coated with ink and the image printed. This cheap and effective halftone process is still used today, mostly in newspapers.