59. About Size and Resolution
60 Change Image Size or Resolution
62 Increase the Area Around an Image
Two of the most common changes you'll make to an image are to adjust its size and to change its resolution. By size, I'm referring to an image's dimensions when printed, not its size onscreen. An image's resolution is determined by the number of pixels (dots) per inch.
To compute an image's print size, the Editor looks at the number of pixels in an image and their relative size (the number of pixels per inch). Take a look at the first figure here, which depicts an image that's ten pixels wide by five pixels high, using an imaginary scale of four pixels per inch. Based on the size of these pixels, the printed image will be about 2.5" wide by 1.25" high.
Figure 1. An (imaginary) image that's ten pixels wide by five pixels high.
To compute the resolution of an image, you simply count the number of pixels per inch. Luckily, Photoshop Elements does that for you, and you can view the image size (such as 2048 x 1536 pixels), resolution (such as 300 DPI), and print size (such as 6.827" x 5.12", which is calculated by taking the image size and dividing it by the resolution) of an image in the Image Size dialog box (choose Image, Resize, Image Size from the menu bar). You'll learn how to use this dialog box to change the size or resolution of an image in 60 Change Image Size or Resolution.
If you never plan to print a certain image, the image won't need a high resolution (a great number of pixels) to look good onscreen. But to print an acceptable image, you need the highest number of pixels you can getthe more the better. As I explain in 64 About Printing Images, you must have an print resolution of 200300 DPI (dots per inch) to create a high-quality print using your home printer; for prints sent off to a lab, 150 DPI usually does just fine.
An image's print size and its resolution are interdependent; changing one without changing the other affects the image's print quality. For example, maintaining the same number of pixels in an image while trying to force it to print in a larger print size decreases the number of pixels per inch and, as you can imagine, inflates the size of each pixel to compensate. If the pixels become too large, a mosaic effect called pixelation might distract you from seeing the image as a whole.
Imagine for a moment that the first figure represents all the pixels in an image whose total resolution is 10 x 5 pixels. As shown in the second figure, if you change the size of the pixels to make them twice as big (in this case, by keeping the same number of pixels while increasing the print size), you'll get a much larger rectangle, but the pixels will be much more apparent. When printed, the rectangle might look more like a mesh of dots than a solid rectangle, which is probably not the effect you're going for. The same is true of any graphic image: If you print the image at something larger than its native print size without also increasing the number of pixels (the image resolution), the pixels will become bigger and more evident in the final print. This is what can happen if you choose to rescale an image "on the fly," using options in the Print Preview dialog box (in the Editor) or the Print Selected Photos dialog box (in the Organizer).
Suppose that, instead of changing the size of the pixels, you maintain their relative size but double their number. In other words, you double an image's resolution (increase the number of pixels) from 150 to 300 PPI, for example, while also doubling the print size of an image. The print quality will remain the same because the pixels will remain the same size, but you'll get a much bigger print, as shown here by comparing the first figure (10 5 pixels) with the third figure (20 10 pixels). The sizes of the pixels are the same in both figures, but the second one is bigger because it has twice as many pixels. Don't need a larger print size, but want better quality? Just increase the resolution (number of pixels) while maintaining the same print size. If you compare figure 2 to figure 3, that's exactly what I've done. The images are the same size, but figure 3 has more pixels and, therefore, more detail and better quality.
Figure 2. Enlarging the pixels increases the image size while decreasing its quality.
Figure 3. You can enlarge an image, and perhaps improve print quality, by increasing the number of pixels.