1. Newspaper Photos Are Made of Tiny Dots, Not Smooth Colors
When you bring a magnifying glass close to a newspaper photo, you can see that human skin and blue skies are actually made of fine dots lined up together. These dots are called halftone dots [1].
Color printing uses four inks: cyan (a bluish green), magenta (a purplish red), yellow, and black. Separate plates are made for each color, and they overlap on the paper as halftone dots [1].
Many home inkjet printers also use these four colors, though some models add more inks. In one experience article, a book cover viewed under a 120x microscope showed that everything except the white areas looked like grains of color [9]. That article did not explain that the grains were halftone dots, but it agreed with the point that printed colors look like grains when enlarged.
But if there are only four colors, how can we see orange or brown?
2. How Four Ink Colors Can Look Like Many Colors
The key is not adding more ink colors, but changing the size of the dots. By slightly varying the size of the halftone dots for each of the four colors, many different colors are reproduced [1].
Printing density is specified from 0 to 100% for cyan, magenta, yellow, and black. Areas at 100% density are solid ink surfaces, while areas below 100% are represented by clusters of halftone dots [4].
Why does making dots create shading? Halftone dots represent an image using a pattern of small dots with limited colors. From a distance, the individual dots cannot be distinguished. For black-and-white (single color) halftones, brightness appears determined by the ratio of dot area to white paper area [6]. Even in black-and-white photos, gray areas are represented by the size of black dots.
This is why lighter areas in newspaper photos show more of the white paper. The ink is not diluted with water; instead, the shade is adjusted by changing dot size.
So, how is the dot size for each of the four colors decided from one original color photo?
3. Color Separation: Splitting a Color Photo Into Four Plates
The process of splitting one color photograph into plates for each of the four colors is called color separation. According to a Canon explanation for children, shooting a color original through three types of filters creates three photos representing the cyan, magenta, and yellow components, which become the basis for those color plates. The fourth plate is a photo that captures only the brightness of the original, which becomes the black (K) plate [2].
Why is a black plate necessary? The same explanation states that if K is missing, the overall tone becomes blurry and vibrancy is lost [2]. The black plate can be said to handle only the brightness of the original.
The reason for splitting into four plates instead of three is that printing creates colors by overlapping layers, known as subtractive color mixing. While displays add light using RGB (additive color mixing), printing typically separates colors into cyan, magenta, yellow, and black [5].
Current plate-making is mostly digital. Here, the method using filters is used as an example to convey the concept of color separation.
This idea is not new. A 1890 patent by Frederick Ives reportedly included a method for making cyan, magenta, and yellow color prints with halftone dots from negatives [8].
Once the four plates are ready, do we just overlap them? Actually, there is a trick to how the dots are arranged.
4. Why the Four Colors of Dots Are Overlapped at Different Angles
The fineness of halftone dots is expressed by the number of dot lines per inch (about 25.4 mm), which is called line count [3]. If the dots from the four plates are overlapped in the same direction, the regular dots interfere with each other, creating unexpected striped patterns. This is called moire.
To make this less noticeable, the angle of the halftone dots for each ink is distributed differently. One source gives an example: cyan at 75 degrees, magenta at 15 degrees, yellow at 30 degrees, and black at 45 degrees [3]. The Japanese Wikipedia generally lists cyan at 15 degrees, yellow at 30 degrees, black at 45 degrees, and magenta at 75 degrees [6]. Since the numbers differ between sources, it is best to view these angles as examples of the concept rather than fixed values.
When overlapped at shifted angles, the dots form small circular or flower-like patterns called rosettes. The English Wikipedia also explains that rotating screens relative to each other reduces moire [7].
The pictures of the four colors of dots are deliberately tilted slightly and overlapped. Knowing this, when you look through a magnifying glass again, familiar patterns begin to appear in the arrangement of the dots.
So, how can we verify this with our own eyes?
5. Try It Yourself: Comparing Newspapers and Books With a Magnifying Glass
Try looking at color photos in newspapers or books you have at home with a magnifying glass. Look at both the color parts and the black-and-white parts. In color, you might see the overlap of four colored dots; in black-and-white, you might see changes in the size of the black dots.
There are safety precautions for observation. Do not look at the sun through a magnifying glass. View objects in a bright room without bringing your eyes too close to the lens.
By observing how dot size varies by location and how the arrangement differs between dark and light areas, you can experience the idea that "shade is created by dot size." Compare this with prints from home printers to observe how dot size and arrangement differ.
You might find dot arrangements in newspapers or books that look like small circles or flowers. It is also noted that DNP held an exhibition explaining the mechanism of creating infinite colors with just four inks [10].