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 milli-meter. 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. In the past, 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. 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.
When enlarging or reducing an image, don’t be afraid to slightly reduce or enlarge an image. But be aware that when an image is scanned or captured by a digital camera, it contains a fixed number of pixels. When you enlarge an image, you’re attempting to generate missing information in a process called interpolation; the result is never as good as a proper-sized original scan. And the more drastic the transformation, the less satisfying the outcome (Figure 4.2).
Figure 4.2 You can’t truly make something from nothing. Notice the loss of detail in the scaled-up version (D) versus the original (A).
Because of the limitations imposed by resolution, it’s helpful if you can anticipate how the image will be used and control photography or scanning accordingly. For typical image content, you can probably scale up to 120–125 percent. If the image is background content without much detail, such as a soft-focus landscape, you have more leeway and can probably get away with scaling up to 150–200 percent. Conversely, if you need to maintain very small details, you may be limited to a maximum of 120 percent.
Photoshop CC introduced a new method that does a better job of scaling up images and upsampling them to higher resolutions—Preserve Details. While the results won’t be equal to a higher resolution original image, it’s a definite improvement over earlier methods (Figure 4.3).
Figure 4.3 If you have to scale up an image, or artificially increase its resolution, be sure to choose the Preserve Details option in the Photoshop CC Image Size dialog box. Tip: If you select “Automatic,” Photoshop CC chooses the best method for resampling.
Scaling down an image also involves interpolation. While the loss of data may not be quite so obvious when you reduce the size of an image, there can be some softening of detail. For best results, choose the Automatic option in the Image Size dialog box in Photoshop CC; it applies some sharpening to camouflage the reduction in detail. While it’s acceptable to scale images in InDesign, if you find it necessary to scale an image below 75 percent of its original size in your page layout, consider scaling it down in Photoshop CC instead, because InDesign can’t sharpen image content.