How Much Resolution Is Enough?
When it comes to image resolution, bigger isn’t necessarily better. The higher the resolution of an image, the longer it takes to open, edit, save, or print. If your output requires only 300 dpi images but you use 1200 dpi photographs in a document just because you can, chances are you’re going to wait longer at every step of your workflow and your hard disks will fill up much faster.
But smaller isn’t necessarily better. If your image resolution is too low, your image will look pixelated (see Figure 2-5): You’ll start seeing the pixels themselves, or adverse effects from excessively large pixels. Loss of detail and mottling are the two worst offenders in this category.
Maybe you thought you could save time by reducing your images to 150 ppi. But if the client rejects the job because the image is too pixelated, any savings are more than wiped out. So if bigger isn’t better, and too small is even worse, how much is enough? How much image data do you need? The first consideration is image mode: The requirements are very different for line art than for grayscale and color.
In general, when printing grayscale and color images to halftoning devices such as platesetters, image resolution should be no more than twice the screen frequency of the halftones. For instance, if you’re printing a halftone image at 133 lpi, the image resolution doesn’t need to be higher than 266 ppi (see Figures 2-7 and 2-8). Under some well-controlled printing conditions and high-quality substrates, it may be possible to see the difference between 2 and 2.5 times the screen frequency, but it will be difficult. When you output an image resolution that’s more than 2.5 times the screen frequency to a PostScript-language output device, you’re basically wasting data: Due to the unnecessary resolution, your images will take more disk space to store and back up, more time to process during editing and production, and more time to transmit across the network or to your printer. Because of this, Photoshop warns you when you try to output an image that’s unusually large.
Figure 2-7 How much resolution do you need? All of these images are printed using the same 133 lpi halftone screen, but they contain different numbers of pixels. Look for details, such as readability of type.
Figure 2-8 Resolution of line art
Many inkjet and laser printers don’t use halftone dots. Instead, they lay down dots using a technique called error diffusion. (See Chapter 12, “Image Storage and Output,” for more on the differences between halftone and diffusion dithers.) These printers use device-resolution dots to build larger areas of color. Many inkjet printers today exceed resolutions of 4000 or 5000 dots per inch, and there is no reason to send those printers an image at half or ever a quarter of that resolution, with the possible exception of line art.
Most of the time we print at between 240 and 360 ppi, depending on the print size and the degree of detail in the image. We’ve gotten good results in the range between 180 ppi for very large prints and 480 ppi for small prints.
For bilevel (black-and-white, 1-bit, bitmap-mode) images, the resolution never needs to be higher than that of the printer you’re using. This is one situation where image pixels per inch equate to printer dots per inch. If you’re printing to a 600 dpi desktop laser printer, there’s no reason to have more than 600 pixels per inch in your image (the printer can output only 600 dots per inch, so any extras just get thrown away). However, when you print to a 2400 dpi platesetter, that 600 ppi image will appear jagged.
If you’re printing line art on press, plan on using an image resolution of at least 800 ppi—preferably 1000 ppi or more. Lower resolutions often show jaggies and broken lines. On newsprint or very porous paper, you may get away with a lower resolution, such as 400 or 600 ppi, because the jaggies will disappear with the spreading ink. But unless you have considerable experience with the print process at hand, you can’t tell until the job has run, so err on the side of caution.
Web and Video
It’s generally misleading to think in terms of resolution when you prepare images for use on screen. All that really matters are the pixel dimensions, such as setting an image to 600 by 400 pixels. The dots-per-inch value varies depending on each individual monitor, so you can’t rely on dpi when sizing for the screen. For example, you can find a 1280-by-900 pixel display on both 13-inch and 15-inch notebook computers, so the dpi value will be different on each display because the same number of pixels covers a different area. In addition, resolution can also change if you change the display settings of a monitor. For these reasons, the old advice that Mac monitors are 72 dpi and Windows monitors are 96 dpi became an obsolete myth a long time ago. Because digital cameras and scanners can produce far more pixels than you need on screen, you typically need to downsample an image before using it on a Web page or other video-based output. (We discuss this in more detail in the next section.)