Photoshop Elements 15 For Dummies. Obermeier Barbara

Image resolution is critical to properly outputting files in the following instances:

      ❯❯ Printing images: The optimal resolution for print is 300 ppi. If the image resolution is too low, the image prints poorly. If the resolution is too high, you waste time processing all the data that needs to be sent to your printer.

      ❯❯ Showing images onscreen: The best resolution for onscreen images is 72 ppi. Onscreen resolution is lower than print to match typical screen resolutions (also called display resolution). Just as images have resolution inherent in their files, your computer monitor displays everything you see in a fixed resolution. Computer monitors display images at 72 ppi (or 85 or 96 ppi or higher). That’s all you get. What’s important to know is that you can always best view photos on your computer monitor at a 72-ppi image size in a 100 percent view.

      Newer devices, such as smartphones and tablets, have screens with higher resolutions. You can find device display resolutions from 150 ppi to more than 300 ppi on a variety of devices. When you design for a specific display, it’s important to know the device display-resolution capabilities before you start working in Elements.

To see how image resolution and screen resolution combine and impact what you see onscreen, look at Figure 2-1. You see an image reduced to 50 percent and then at different zoom sizes. When the size changes, the monitor displays your image at different resolutions. For example, if you view a photo with a resolution of 72 ppi and reduce the size to 50-percent view on your monitor, the resolution on the monitor appears as though the photo is at 144 ppi. When the size is 100 percent, the image resolution is the same as the monitor resolution. Table 2-1 provides a closer look at these differences in resolution.

       FIGURE 2-1: The same image is viewed at different zoom levels.

TABLE 2-1 How Image and Display Resolutions Affect What You See Onscreen

      This relationship between the image resolution and viewing the image at different zoom levels is an important concept to grasp. If you grab an image off the web and zoom in on it, you may see a view like the 800-percent view shown in Figure 2-1. If you acquire a digital camera image, you may need to zoom out to a 16-percent view to fit the entire image in the image window.

      The reason that these displays vary so much is because of image resolution. That image you grabbed off a web page might be a 2-inch-square image at 72 ppi, and that digital camera image might be a 10-x-12-inch image at 240 ppi. To fill the entire window with the web image, you need to zoom in on the file. When you zoom in, the image appears as though it’s reduced in resolution.

       When you zoom into or out of an image, you change the resolution as it appears on your monitor. No resolution changes are made to the file. The image resolution remains the same until you use one of the Elements tools to reduce or increase the image resolution.

       Understanding image dimensions

      Image dimensions involve the physical size of your file. If the size is 4 x 5 inches, for example, the file can be any number of different resolution values. After the file is open in Elements, you can change the dimensions of the image, the resolution, or both.

      When you change only the dimensions of an image (not the number of pixels it contains), an inverse relationship exists between the physical size of your image and the resolution. When image size is increased, resolution decreases. Conversely, when you raise resolution, you reduce image size.

      The Art of Resampling

      In some cases, images are too large, and you need to reduce their resolution and physical size. In other cases, you might need a higher resolution to output your images at larger sizes. This method of sizing – changing the size, as well as the number of pixels – is dubbed resampling an image. Specifically, reducing resolution is downsampling, and raising resolution is upsampling.

      Here’s a quick example to clarify the benefit of resampling. Say you have a photo taken with a digital single-lens reflex (SLR) camera, which takes pretty high-resolution photos. The photo could easily have a resolution of 300 dpi and dimensions of 14 x 10 inches. If you just want to make a few color corrections in Elements and then make 7-x-5-inch prints for your family members, resampling the file keeps the file size manageable for both your computer’s memory and for uploading to a printing service. Because you decreased the photo’s dimensions, the photo still maintains the resolution you need for a high-quality photo print (that is, 300 dots per inch; dpi). If you just changed the photo’s dimensions without resampling, the photo’s resolution would almost double, making for an unnecessarily large file.

       Use caution when you resample images; when you resample, you either toss away pixels or manufacture new pixels. To protect your images during resampling, work on a copy of your image, instead of the original file.

       Changing image size and resolution

      You can change an image’s size and resolution in a couple of different ways. One method is cropping images. You can use the Crop tool with or without resampling images. For more information on using the Crop tool, see Chapter 9. Another method is using the Image Size dialog box, which you use in many of your editing sessions in Elements.

      To resample an image with the Image Size dialog box, follow these steps:

      1. Choose Image ⇒ Resize ⇒ Image Size.

Alternatively, you can press Ctrl+Alt+I (⌘ +Option+I on the Mac). The Image Size dialog box opens, as shown in Figure 2-2.

      The Pixel Dimensions area in the Image Size dialog box shows the file size (in this example, 12.1M). This number is the amount of space the image takes up on your hard drive. The width and height values are fixed unless you select the Resample Image check box at the bottom of the dialog box.

      2. In the Document Size area, redefine dimensions and resolution.

      The options are

      ● Width: Type a value in the text box to resize the image’s width. Then press Tab to move out of the field and implement the change. From the drop-down list to the right of the text box, you can choose a unit of measure: percent, inches, centimeters, millimeters, points, picas, or columns.

      ● Height: The Height options are the same as the Width.

       If you keep the sizing proportional, you typically edit either the Width or Height text box, but not both. When you alter either width or height, the resolution changes inversely.

      ● Resolution: Type a value in the text box to change resolution and then press Tab to commit the change. After you edit the resolution, the Width and Height values are changed inversely (if the Constrain Proportions check box is selected).

      3. (Optional) If you’re okay with resampling your image to get the desired size, select the Resample Image check box.

      With this check box selected, you can change dimensions and pixels at the same time, which results in either reducing or increasing the number of pixels. When the check box is deselected, the values for dimensions are linked; changing one value automatically changes the other values.

      Before you resample your image, however, be sure to check out the following section, “Understanding the results of resampling.”

      4. (Optional) If you select the Resample Image check box, choose a resampling method from the drop-down