More about prepress

To adjust the resolution of the pictures, we need to take a look at things backward meaning that we should examine the production, which plays a key role in printing, first.
In lithography, graphic files are converted to a matrix of dots. The density and the angle of these dots can affect the final quality of the file. It is the printing material and the standards of the printing machine which determine the quality of converting a picture to dots. The designer’s job is to determine the quality based on the resolution defined in the lithography to save time during the reproduction and change the file to the dots.

Let us clarify the matter with a few examples:

you are designing a newspaper for two employees. The first employee’s newspaper will be printed with an Offset Web machine on bulk paper, an ordinary low-quality paper. This paper cannot receive a lot of ink. It is made to be used for works of average quality, and it is not very costly. On the other hand, Offset Web machines have a high-speed rate, and they cannot drain a big portion of ink quickly.
The second employee wants to print on high-quality papers such as coated papers with the Offset Sheetfed machine in low circulation. Unlike bulk papers, this printing material has more quality and can support more ink. In this paper, you can print pictures with bigger sizes; and additionally, you can print colors with more accuracy. Meanwhile, Offset Sheetfed machines have a low rate of speed, support low circulation printing, but their production is more costly than the former machine.

Do the quality of the printing material and the type of printing machine affect the resolution of the picture? The answer is YES

Lithography analyzes your file considering the printing material and the printing machine. The ink endurance of the printing material causes a change in the quality of the dots which is created out of the file.
Lithography uses resolution modules as well; however, it uses them in determining the quality of dots. In this usage, the density of dots alters regarding the production type.
In the mentioned example, bulk papers cannot take high-density dots; therefore, a low resolution must be used. If you do not use low resolution, the high density of dots will cause extra ink intake leading to the possible damage of paper and severe color change while printing.

what if...?
If the designers try to adjust the resolution of the picture without knowing about LPI in lithography, they have only increased the size of the file aimlessly. And this will make the file analysis complicated during the lithography.

The lithography machine uses a laser to produce dots, and it arranges them in a 1inch unit. Dots seat next to each other shaping a line to simulate part of the picture. Resolution in lithography is measured with LPI (line per inch) unit.
The higher the quality of the printing material and the printing machine, the more you can raise the LPI so that the density of dots is increased and the final print is more accurate.

SOLUTION

Now we take a look at the unit of graphic designing. Graphic software such as Adobe Photoshop uses a unite called Pixel to determine the color and light of the picture. By putting together pixels in 1inch distance, a complete picture is created; however, we have to determine the number of pixels concerning our production needs. With this practical formula, we can find out the relation between resolution in lithography and graphic software:

LPI x 2=PPI

To get the maximum PPI of a picture, you need to multiply the number that lithography uses to determine the quality of dots by 2. PPI is the number of pixels inside a square with an edge of 1 inch. For example, when we say 2PPI, it means that there are 2 pixels within the length and width of the square, and overall 4 pixels within the square; consequently, when we say 10PPI, it means there are 10 in the width, and 10 in the length; and overall there are 100 pixels within the square.
When the resolution is raised to 300PPI, we have exactly 90000 pixels in 1x1 inch square. Now see how many pixels are in an A4 paper that contains so many 1 inches.
Number 2 in the quantization formula (LPI*2=PPI), determines a proper quality for the resolution of pictures and eventually the quality of professional printing. These measures can be determined by the formula Draft=1 or Good=1.5 too; however, since nowadays quality is a priority for the employees, the measure Best=2 is used frequently. So if we want to determine the PPI, we should simply ask the lithography about the LPI and then multiply it by 2.

Final point: you might find designers who, unlike lithography’s expectations, would use 300PPI even for ordinary printing papers. But the thing is, in this situation, the usage of the ink increases; therefore, to avoid damaging the surface of the paper, the printer’s operator will decrease the input of the ink, ultimately causing color saturation.
On the other hand, designers, by considering the right amount of resolution, help to decrease the waste and also save the time spent on production. There are so many details and points regarding the adjustment of color and print quality, and resolution is only a small part of the Prepress world.

In Offset Sheetfed prints, due to its simple procedures, we choose 150 or 175 for LPI to get a high-quality print leading to the easier calculation of PPI.