Optimal resolution for files
Let's talk about the magic numbers that influence image quality and resolution. These details, while often overlooked, can significantly enhance your design and printing processes. Beyond the theoretical aspect, these concepts have real-world applications. Before we delve deeper, it's crucial to understand the difference between print resolution and file resolution. A basic grasp of halftones and printing processes is essential.
Now, let's trace the journey of your file through the printing process. By understanding this process, we can determine the optimal resolution for your file.
Once saved, your file is sent to the prepress department. The first thing to remember is how the prepress department will prepare the file for printing. Using RIP software, they analyze the digital file and convert it into halftone dots. This process is called mapping.
Each of these tiny dots or halftones is part of a matrix or grid created by the RIP. The finer the grid, the more accurate the final halftone. The squares that make up each grid in a halftone are called dots, and the laser's resolution is measured in dots per inch (DPI). As discussed in previous articles, there's a formula linking DPI to PPI (pixels per inch). So far, we've explored two quality-related units: DPI and PPI.
Creating the Dots
As previously mentioned, the digital pixels in a file are replicated by the RIP and then simulated by a laser onto a printing plate. But here's a crucial question: How many of these dots does the laser need to create? Intuitively, more dots would result in a higher quality halftone. However, is there a standardized number, and if so, what is it?
Similar to our in-depth exploration of Bit Depth, let's conduct a thought experiment about pixels. Imagine embarking on a journey into the world of pixels. Let's assume we have a set of pixels that the lithography laser will replicate using minuscule dots. The space occupied by a halftone is called a halftone cell, and the individual dots within it are known as microdots. (We'll delve deeper into halftone shapes in future discussions.)
Let's start by simulating the halftone's pixels using four units. We can then increase this to eight units and continue this process until the halftone quality is satisfactory. We find that 16 units yield optimal results. With 16 microdots per row and column, we arrive at a total of 256 units, which is the golden number for the human eye to perceive grayscale levels.
Introducing LPI
Halftones are arranged at different angles to create a printed image. These columns form another unit called LPI (lines per inch). The more columns, the smaller the halftones. LPI measures the number of columns in an inch and is determined by the RIP. Designers cannot adjust this value directly; it's controlled by the lithography department for each color.
The three core resolution units—PPI, DPI, and LPI—are interconnected. But what do they do? The exciting part is that they connect the designer directly to the printer and prepress department. Based on machine capabilities, paper type, graphics, and production process, the printing press will specify the appropriate LPI. A common value is 150 LPI.
Using the previous calculations, we get 150 × 16 = 2400 dots. So, the laser in the lithography machine must have a minimum resolution of 2400 DPI.
How does this help me?
This information becomes relevant in Photoshop's Image Size dialog box. If you select "Auto Resolution", you'll be asked to enter the screen frequency based on LPI or LPC. Here, you would input 150. Then, choose the "Best" quality option, which typically corresponds to 300ppi.
The "Quality" setting is a multiplier for LPI, determining the final PPI. This factor can vary based on factors like printing press capabilities, laser resolution, paper type, and halftoning method.
The common assumption of setting resolution to 300ppi is more complex than it seems. It's not just a preference; it's based on the intricate relationship between the design, printing process, and the capabilities of the printing equipment. Inputting higher resolution values unnecessarily can hinder the production process. Therefore, understanding these three resolution units and their interconnections is crucial for optimal results.
