The previous section explained how to resize your view of an image—how it appears on your monitor. But sometimes you need to change the actual size of your image, and that’s what this section is about.
Resizing photos brings you up against a pretty tough concept in digital imaging: resolution, which measures, in pixels, the amount of detail your image can show. What’s confusing is that resolution for printing and for onscreen use (like for email and the Web) are quite different.
You need many more pixels to create a good-looking print of an image than you do to view the image clearly onscreen. A photo that’s going to print well almost always has too many pixels in it to display easily onscreen, and as a result, its file is usually pretty hefty for emailing. So you often need two copies of your photo for the two different uses. If you want to know more about the nitty-gritty of resolution, a good place to start is www.scantips.com.
This section gives you a brief introduction to both onscreen and print resolution, especially in terms of what decisions you’ll need to make when using the Resize Image dialog box. You’ll also learn how to add more canvas (more blank space) around your photos, which you’d do to make room for a caption below your image, for instance, or when you want to combine two photos.
To get started, open a photo you want to resize and then go to Image→Resize→Image Size or press Alt+Ctrl+I/Option-⌘-I. Either way you should see the Image Size dialog box shown in Figure 3-20.
Figure 3-20. This dialog box gives you two different ways to change the size of your photo. Use the Pixel Dimensions section (shown here) when preparing a photo for onscreen viewing. (The number next to Pixel Dimensions—here, 28.6M—tells you the current size of your file in megabytes or kilobytes.) Before you can make any changes here, you have to turn on the Resample Image checkbox at the bottom of the dialog box (not visible here), since changing pixel dimensions always involves resampling (see page 121).