If you’ve ever found yourself staring at the cereal box too long, or have walked past a billboard advert at eye level, you may have wondered how, or why it was that the images were made up of tiny dots, that would normally be invisible to the naked eye when viewed from afar. If on the other hand, you’ve never seen this odd phenomenon before, make haste! Head for your kitchen this instant and begin to examine the packaging of your most coveted produce. You may have to strain a little, particularly if you are a fan of Marmite, because the dots of which I speak come in all sizes, and even shapes.
The reason for the existence of these dots is that most mechanical printing processes are only capable of reproducing images that are formed in a binary fashion. Either there is a dot of ink on the paper, or there is not, so in order to produce a series of tones to give the impression of a smooth gradation from light to dark, there has to be a method for applying dots of varying size or frequency. In this way, we can create the illusion of a grey tone, by using black dots that are interspersed with a series of white gaps. This is different from chemical photographic printing, where sensitised paper can be exposed and developed to different degrees.
The photo above also gives us another clue about commercial printing, which is that just as the illusion of tone is created by different-sized dots, the entire gamut of colours that you perceive when viewing most printed media is also an illusion, because every different colour is essentially composed of cyan, magenta and yellow in varying amounts.
There exist continuous-tone photomechanical printing processes which were once used to produce commercial work, such as images for books and postcards, however, these almost became extinct once the problem of transforming a photographic image to a series of dots was accomplished, which allowed photo-realistic imagery to be printed mechanically at high speed, with great reproducibility, and at lower cost. Such is the history of photography.
To turn a continuous-tone image or subject into a series of dots, a piece of equipment called a halftone screen was used. This screen was essentially a series of lines forming a tightly knit grid, that allowed light to pass through the centre of each square. Each opening of the grid therefore functioned as an individual aperture for the tiny section of image that it would transmit, and in this way, each aperture allowed a corresponding amount of light through, which in turn created dots of different sizes. This is because the more light that passes the screen, the more light is able to spread out from the centre of the aperture to the areas towards the edges, thus the different dot size is a demonstration of the vignetting effects seen with pinhole cameras and lenses with small image circles. The distance from the top of the halftone screen to the centre of the dot on the other side of the aperture is significantly shorter than the distance to the extreme edges. The closer the light source to the subject, or in this case, the aperture to the image plane, the greater the light fall-off and the more pronounced the vignetting.
If we were to move the halftone screen further away from the image plane, the dots would become softer and less pronounced, eventually showing as well-defined image areas rather than monotone dots. At a great enough distance, any image projected through a screen such as mosquito netting for example, would not show evidence of the screen, because the distance that the light would have to travel between the inner and outer edges of the gaps would be negligible.
William Henry Fox Talbot was one of the first people to experiment with halftone screens, using materials such as silk cloth, which already has a built in grid pattern. Later on halftone screens were made on glass and on film, and nowadays the effect is recreated in Photoshop. In fact, if you search for how to make a halftone image or a halftone screen, you are unlikely to come up with anything other than tutorials demonstrating manipulation of digital images using software. It now seems a simple matter of ‘you push the button we do the rest‘ when it comes to creating halftone images, therefore anyone silly enough to want to do it the old-fashioned way, or as I prefer to think of it; as doing it myself, rather than relying on someone or something else to do it for me, must almost re-invent the wheel, or at least re-discover it through trial and error. History is so much more than written accounts, but unless effort is made to preserve practical skills and intangible cultural heritage, the only place where these things will exist is in books.
My search for how to make a traditional halftone image began with the desire to experiment with new processes that require halftone images, and to understand how they work. With the relatively rare few pieces of information I was able to find on the internet I decided to set about working through trial and error.
Without knowing it at the time, my first attempts at re-creating a halftone screen resembled Fox Talbot’s early work, except that I used the dark cloth for my large format camera which seemed perfectly symbolic.
For the first test, I contact-printed a sheet of film through the cloth at different time intervals to get a sense of what the effect was, and where the correct exposure may lie.
The results were positive, yet puzzling: why were the dots seemingly grouped in patches that resemble a grid pattern itself? Later I realised that due to the thickness of my dark cloth material, I had doubled it over in order to make it as opaque as possible, which seemed to result in this uneven exposure. After further, unrelated research, it appears that the two layers of cloth created a moiré pattern, resulting in ‘clumps’ of dots rather than a more uniform covering.
After the initial test I exposed a new sheet of film and developed it, and then made a copy of it.
As you can see the dots differ in size due to the uneven spacing of the cloth fibres, and the areas of greater exposure have resulted in dots that bleed into one another. My aim was to produce a dotted screen which would then be used in contact with a continuous-tone film negative to produce a halftone positive when the ensemble is exposed to paper or film, and developed.
As you may be able to tell from the example above, the dots from my screen appear small enough, but there is too much space between rows of dots, which creates a lot of empty space.
In the image above a close-up of a photo from a magazine can be seen side-by-side with part of a photograph made with my own screen. Notice how the dots appear in uniform rows in both examples.
Here’s the result of exposing a 4×5 inch negative through my very crude halftone screen. Although this was a test strip given different exposures, it’s enough to see the type of effect we can expect this early on in the process. Much more work is needed, but at least I now have a better understanding of the history and technique, and should be able to make progress quickly with a few adjustments.