Resolution and Image Fidelity

The resolution of an image is generally measured in pixels per inch (ppi) unless you speak metric, in which case it’s expressed in pixels per millimeter. Determining the proper resolution for Web images is simple: 72 ppi at final size. But there are strongly held (and hotly debated) beliefs regarding the appropriate image resolution for printing. Some hold that 150 percent of the final screen ruling value is sufficient, and some believe twice the final ruling is preferable, largely because it’s easier to calculate the resolution. For example, an image that will be printed at 150 line screen should have a resolution of 300 ppi. When typical hard drives held 80 MB, networks were glacially slow, and RIPs choked on 15 MB PostScript files, it was important to trim off every little bit of fat, so we agonized over resolution. But now, with hard drives measured in hundreds of gigabytes, and RIPs with much more robust digestive tracts, we can afford the luxury of a few extra pixels. That said, there’s rarely an advantage to exceeding 300 ppi, except in some cases for higher line screens such as 175 lpi printing. So put away the calculator. For most circumstances, 300 ppi at final size is adequate and provides a bit of elbow room if you have to slightly reduce or enlarge an image.

But you do have some leeway, depending on the nature of the image and how it will be used. For example, a gauzy, soft-focus shot of a sunset that will be used as a ghosted background accent in a magazine can be used at 200 ppi with no problem. A highly detailed close-up image of an important piece of antique jewelry in a 175 lpi art book should be at 300–350 ppi. At the other end of the spectrum, an image for use in an 85 lpi newspaper can be 130–170 ppi, because much of the information in a 300 ppi image would be lost when printed in the coarse newspaper screen ruling. So you might consider the determination of appropriate resolution to be an equation based on image content and the final printing line screen rather than an absolute number.