Resolution is one of the most overused and least-understood words in desktop publishing. People use it when they talk about scanners and printers, images and screens, halftones, and just about anything else they can get their hands on. Then they wonder why they're confused. Fortunately, resolution is easy once you get the hang of it.
As we noted earlier, an image in its pure digital state has no physical size—it's just a bunch of pixels, and the pixels can be any size you want. So whenever you give an image tangible expression, whether on the screen or in printed form, you confer upon it the property of physical size, and hence resolution.
The resolution of an image is the number of pixels per unit of measurement—usually the number of pixels per inch (ppi) or pixels per centimeter (ppcm). If your image has 72 pixels wide and you tell it to be 72 pixels per inch, then it's an inch wide. If you print it at half the size, you'll still have the same number of pixels, but they'll be crammed into half the space, so each inch will contain 144 of them. Or, if you take the same image and change its resolution to 36 pixels per inch, suddenly the image is two inches wide (same number of pixels, but each one is twice as big as the original; see Figure 3-4).
Figure 3-4 Scaling and resolution
You can also look at resolution in another way: If you know an image's size and resolution, you can figure out its dimensions. When you scan a picture that is three inches on each side at 100 pixels per inch, you know that the image has 300 pixels on each side (100 per inch). If you scan it at 300 pixels per inch, the dimensions shoot up to 900 pixels on each side.
The key to making resolution work for you is knowing how many pixels you need for the intended purpose to which you'll put the image.