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As often as not, you’ll have no desire to change the number of pixels in an image; you’ll just want to change how it looks on the printed page. By focusing exclusively on the resolution, you can print an image larger or smaller without adding or subtracting so much as a single pixel. Doing so means you aren’t throwing away any valuable data that you might want later.
For example, let’s say you want to scale this portrait so that it prints 5 inches wide by 7 inches tall. If you press Image→Resize→Image Size in either Photoshop or Photoshop Elements, you would see that it's currently set to print at 3.117 x 4.364. To make a bigger print, would you upsample the image and thereby add pixels to it?
Absolutely not. The Image Size command can’t add detail to an image; it just averages existing pixels. So upsampling adds complexity without improving the quality. Upsampling can be helpful—when matching the resolution of one image to another, for example—but these occurrences are few and far between.
The better solution is to modify the print resolution. Start by choosing Image→Resize→Image Size and turn off the Resample Image checkbox. Note in this example that the Pixel Dimensions options are now dimmed and a link icon joins all three Document Size values.
The chain link icon tells you that it doesn’t matter which value you edit or in what order. Any change you make to one value affects the other two, so you can’t help but edit all three values at once. For example, change the Width value to 5 inches. As you do, the software automatically updates the Height and Resolution values to 7 inches (because those are the proportions of the original image) and 187 ppi, respectively. So there’s no need to calculate the resolution value that will get you a desired set of dimensions; just enter one of the dimensions and Elements does the math for you.
(Note that anything over 150 ppi resolution will probably be adequate for consumer desktop printers, but not every image has enough data to be printed as large as you might like.)
Click OK to accept your changes. The image looks exactly the same as it did before you entered the Image Size dialog box. This is because you changed the way the image prints, which has nothing to do with the way it looks on-screen. If you like, feel free to save over the original file. You haven’t changed the structure of the image; you just added a bit of sizing data.
Note: Contrary to what you might reasonably think, print resolution is measured in linear units, not square units. For example, if you print an image with a resolution of 300 pixels per inch (ppi for short), 300 pixels fit in a row, side-by-side, an inch wide. In contrast, a square inch of this printed image would contain 300 × 300 = 90,000 pixels.
To learn more about printing—including how you can further modify the print resolution from the Print Preview dialog box—check out Adobe Photoshop CS4 One-on-One or the upcoming Photoshop Elements 8 One-on-One.
To hear Deke wax poetically on resolution, here's a link to part 1 of a conversation we had on resolution on Martini Hour (our weekly digital imaging podcast):
Martini Hour 025, In Which Colleen Experiences Deke in High Resolution
